1. Humidity and Temperature Objectives page 21 Introduction page 21 What is relative humidity? page 21 How does relative humidity change? page 22 Why worry about relative humidity and temperature? page 22 Can the damage be prevented? page 25 Identifying the source of the problempage 25 What can be done to minimise damage? page 26 Modifying the conditions in buildingspage 26 A word of caution about travelling exhibitions and loanspage 29 In an ideal world page 30 Australias climatic zones page 30 MORE ABOUT RELATIVE HUMIDITY AND TEMPERATURE Measuring relative humiditypage 32 Dehumidifierspage 34 Humidifiers and steam generators page 34 The use of silica gelpage 35 For further readingpage 35 Self-evaluation quiz page 35 Answers to self-evaluation quizpage 37

2. ObjectivesSteps can be taken to minimise fluctuations inrelative humidity and temperature, and to protectvaluable collections from the adverse effects of At the end of this chapter you should: extremes of relative humidity and temperature. Todo this, it is useful to understand how relative know some of the adverse effects that humidity and temperature are related, what causesextremes and fluctuations in relative humidityrelative humidity to fluctuate, and to know whatand temperature have on objects;effects they have on different types of materials. understand the relationship between relativehumidity and temperature; andWhat is relative humidity? be able to take steps to limit damage tocollections caused by fluctuations in relativehumidity and temperature. Heat really needs no explanation. You may havedifficulty defining it, but youre certainly familiarwith the experience of it. Relative humidity, onthe other hand, does need explanation.IntroductionWater is an extremely pervasive substance and can Relative humidity and temperature are essentialbe found everywhere on the planetincluding in components of a comfortable working environment. the air, where its held as vapour. However, there can be some conflict between the needs of people and the requirements for the careThe capacity of air to hold water-vapour varies of collections.according to the temperature of the air.Relative humidityRHis the amount of waterThe warmer the air, the more water-vapour it can vapour contained in the air at a particularhold. As the air cools down, its capacity to hold temperature compared with the total amount ofwater will decrease. water vapour the air can contain at that temperature. Relative humidity is expressed as a Relative humidity is a measure of the amount of percentage. Various materials respond differentlythe amount of water-vapour contained in air at a over a range of humidity levels and there is anparticular temperature. It is basically a comparison optimum level of RH suitable for the display and between: storage of mixed materials. the amount of water-vapour held in the air at Extremes of relative humidity and temperature canany one time and at a particular temperature; adversely affect the condition of objects, and particularly those made of organic materials. But it the total amount of water-vapour which the is important to remember that many materials willair can hold at the same temperature, that is, stabilise in a particular climate, even thoughthe amount of water which will saturate the conditions are extreme. Once stabilised andair at that temperature. conditioned to the climate, these materials will not necessarily be adversely affected by the Relative humidity is expressed as a percentage. constant, extreme conditions.This can be written as an equation: If the climate changes or objects are moved to a RH = water-vapour present in the air x 100% different climate, problems can arise. Changes in relative humidity and temperatureparticularlywater-vapour required to saturate rapid changesare potentially far more damagingair at that temperature to most materials than are constant extremes. These changes can occur easily with sudden As the temperature of air increases, its capacity to changes in the local weather, when air-contain water-vapour increases. For example: conditioning units break down, and when objects are moved from storage boxes to open display or At 0C the air can hold about 6 grams of water from one region of the country to another. for each cubic metre of air, that is, 6g/m3. Humidity and Temperature 21 3. At 10C this increases to 10g/m3This is an important concept, because display cases and sealed storage areas in some waysAt 20C it increases to 17g/m3behave like sealed boxes; and the relative humidity can vary because the temperature varies.And at 30C it increases to 30g/m3 This principle can also be applied to some extentSo, if air at 20C contains 8.5g/m of water-vapour:3 to museums, galleries and libraries. But they are generally far more complex. RH = 8.5 x100% They have doors which admit moisture-containing17 air from the outside, where the temperature and= 50%relative humidity is usually different from the inside conditions.Thus the relative humidity would by 50%. People come into these areas. They raise the temperature, especially when they are in large How does relative groups; and they take in and give out moisture as they breathe. humidity change? Museums, galleries and libraries contain objectsRelative humidity is a measure of the amount ofwhich take up water and give out water, accordingwater-vapour contained in air at a particularto the temperature of the surrounding air.temperature. The capacity of air to hold water-vapour varies according to the air temperature.There may also be heating and cooling devices and/or air-conditioning in the building.However, although raising the temperature increasesthe capacity of air to hold water, there is not alwayswater available to move into the air to fill that Why worryincreased capacity. So changes in temperature oftenlead to quite significant alterations to the relative about relative humidityhumidity.and temperature?For example, in an empty, sealed box containing afixed amount of water-vapour, raising theRelative humidity and temperature are two of thetemperature will lower the relative humidity. This environmental factors which can contribute to theis because the capacity of the air to contain waterdeterioration of our valued collections.has increased but the actual amount of water hasremained the same. Using our previous example ofair at 20C with 8.5g/m3 of water-vapour, if thetemperature is raised to 30C and no additionalwater is available: at 20C RH = 8.5 x 100%BUT at 30C RH = 8.5 x 100% 17 30 = 50% = 28.3%The reverse is also true. If the temperature in asealed box continuing a fixed amount of water-vapour is lowered, the relative humidity will A bark paintingsplit after drying out in a lowincrease. The capacity of the air to hold water has relative humidity environment.decreased but the amount of water has remainedPhotograph courtesy of Artlab Australia, reproducedthe same. with permission of the Museum of Victoria 22 Humidity and Temperature 4. Extremes of temperature and relative humidityincreased biological activity. Most insects and and rapid fluctuations in thesecan lead to amoulds thrive and reproduce readily in warmer range of problems. The risks of physical damage, conditions; and such as warping, cracking and splitting, chemical deterioration, and insect or mould attack are all acceleration of chemical deteriorationprocesses. Temperature affects the rate at increased when temperature and relative humiditywhich chemical reactions take place. For are too high or too low. example, a temperature rise from 2030C maydouble the rate of some degradation reactions.And this worsens if light, water or pollutionalso contribute to these chemical reactions.For more informationFor more information about how light and pollutants can affect chemical deterioration, please see the chapters on Light & UV Radiationand on Dust and Pollutants in this volume.The discolouration of the cotton proceeded muchmore rapidly in a damp environment. Fluctuations in temperature cause:Photograph courtesy of Artlab Australia expansion and contraction. If this is unevenand/or rapid, it can cause physical damageand distortion. This can be hazardous forobjects made of composite materials; andsome types of plastic, for example, vinylrecords, shrink and warp in high temperatures.The most important effect of temperature is the effect it has on altering relative humidity levels.The effects of extremes andfluctuations in relative humidityIn high relative humidity conditions insects andInsects generally like a warm damp environment.moulds thrive and reproduce readily, metals corrode, dyes and textiles fade and deterioratePhotograph courtesy of Artlab Australia more quickly, organic materials such as wood and leather swell or change shape, and gelatine emulsions and adhesives become sticky.For more information For more information about insects and mould, please see the chapter on Biological Pests in this volume. The effects of extremes and fluctuations in temperatureExtremes and fluctuations in temperature are potentially less harmful than extremes or fluctuations in relative humidity; but it is difficult to High relative humidity conditions promote the separate the two because they are closely corrosion of metals. interrelated. Some independent effects of high Photograph courtesy of Sarah Jane Rennie temperature include: Humidity and Temperature 23 5. contract. Rather, it will compress, leading to cracking or separation between canvas and paint layer.Different components of single objects absorb moisture at different rates and swell by different amounts. This can cause problems, such as paint layers splitting and separating from timber panels.Composites of metal and wood are affected also. As the metal corrodes, the wood starts to split in order to accommodate the corrosion products.Adhesives that absorb water become sticky and are an attractive food source for moulds and insects. The bolts are corroding in high relative humidity conditions. The corrosion products are staining the wood, and will eventually cause splitting.Gelatine emulsions on photographs also swell in humid conditions and can readily stick to the glass Photograph courtesy of Sarah Jane Renniein their frames or, if they are stacked, they can stick together.Organic materials absorb water. This is particularlynoticeable in thinner materials, such as paper,As for papers which are stuck down at the edges,vellum and parchment, textiles, leather and bark they will increase in size in humid conditions andpaintings. As materials absorb water, they swell thus expand in the middle as their edges areand change shape, for example, stretched vellums restricted. This can lead to creasing.and mounted textiles sag. In very low relative humidity conditions, such asThe effects of humidity on organic materials are in arid areas:not always immediately noticeable. But after awhile, extensive damage eventually occurs. insects can still survive;organic materials give out the moisture theyFor example, a large block of wood may take weeks contain. This can cause materials to dry outor even months to transfer water from its surface and become brittle or to distort and split;into its bulk, leading to different parts of thewood having different water-contents. Thethicker materials lose moisture much moreconsequence this has on the wood is to make itrapidly from their surface. This can causeswell by different amounts, which will have the warping;effect of splitting and warping the material. different components of single objects releasemoisture at different rates, which can causeWood also swells more across the grain than along the bonds between them to loosen; andthe grain; and by an amount which variesaccording to the type of wood. This makes life adhesives dry out and crack, and can fail ascomplicated when caring for furniture.a result. Textiles can display what seems to be the opposite If fluctuations are occurring constantly, theresponse to changes in relative humidity. A multi- materials are being subjected to constant movementstrand thread shortens in length when the relative which is usually not uniform and often results inhumidity goes up. This is because the individual cracking, splitting and warping. Some examples ofthreads expand in diameter more than they expand extreme damage caused by fluctuations are:in length. The result is that the strands wraparound each other more tightly, which causes the bark paintings expand and contract as theyabsorb water and release it. This leads to theoverall length to decrease. Often this process does bark warping and splitting, and they can losenot reverse when the relative humidity drops again. paint;A canvas responds in the same way as textiles; bone and ivory are very susceptible tohowever, the paint layer on the canvas does not damage caused by fluctuations; and they warp 24 Humidity and Temperature 6. and split. This is especially a problem for very Identifying the sourcethin ivory sheets, such as those used forminiature painting; of the problem furniture with veneers can be damagedseverely, because the thin, veneer layer isExtremes and fluctuations in relative humidity andlikely to curl and pop off the surface of thetemperature which damage collections arefurniture if it repeatedly expands and experienced in many museums, galleries andcontracts; and libraries. These changes can be caused by: fluctuations in relative humidity can also alter the regional climate;the chemical composition of some minerals,so that they become another mineral. the climate within buildings; What happens in extreme, localised climates with buildings;but stable environments? microclimates; andWhen conditions are extreme but constant, damagevisitors. can still occur. Experience shows, however, that many materials become conditioned to an extreme Australia is a large country with three very environment.different climatestropical, arid and temperate. Regional climates are particularly significant for An object in constantly high or low relativeobjects which are displayed outside. humidity does not absorb and lose water Because buildings are not fully sealed, outside repeatedly; and it is not subject to the enormous conditions have a significant influence on the stresses of the cycles which affect objects in climate inside the building. The buildings style, fluctuating environments. Such an object is likelythe materials used, the state of repair of the to be preserved longer and in better conditionbuilding and whether the building is insulated, air- than a similar object in a fluctuating environment. conditioned or without either, all influence the impact outside conditions have on the climate Remember, the emphasis should be on stability.inside the building.Although buildings are not fully sealed from the Can the damageoutside weather, they act as barriers to the free flow of heat and moisture. This is why air-be prevented?conditioning and heating are effective in providing a comfortable climate. Damage to objects and collections cannot always Within buildings there are localised climates and be prevented totally; but it can certainly be microclimates where conditions vary greatly from limited and slowed by controlling the relative conditions in other parts of the building. For humidity and temperature. example, there would be greater variations in temperature and humidity in a small tea room with The most significant effect temperature can havea toaster and a kettle than there would be in a in a museum, gallery or library environment is theclosed-off storage area. way it can alter relative humidity levels. Cupboards, display cases, boxes and frames act as Relative humidity and temperature are closely barriers to air and moisture circulation and can linked; and it is helpful to understand this link develop their own microclimate. The materials used when setting out to control the environment where to make the display cases, boxes and frames, collections and items of value are stored.combined with whats stored in them, also influence the microclimate. Remember that for a fixed quantity of moisture in a given air space, as temperature rises, the relative People are important to the museums, galleries humidity drops and, as the temperature drops, the and libraries. The effect they have on the local relative humidity rises.climate depends on:Humidity and Temperature 25 7. how many visitors there are and whether they There are two basic types of cooling arrive individually or in groups;airconditioners available.whether they have wet or damp umbrellas andAn evaporative airconditioner works by passing air coats;over a moist surface and increasing the moisturehow long they stay; andcontent of the air, raising the relative humidity.This type of airconditioner should not be usedtheir agesschool groups are potentially unless there is a dehumidifier to remove the more disruptive to a controlled environmentmoisture from the cool air. than adult tour groups or individuals.Cooling coil airconditioners work on theAll of these factors can contribute to fluctuations refrigerator principle of keeping the air cool andin, and problems with, relative humidity. Careful dry. This type of airconditioner should be usedmanipulation of these factors helps create awith caution, and preferably with a humidifier tostable environment where the risk of damage add moisture to the air. Monitoring theis minimised. effectiveness of such equipment is crucial. If you have airconditioning or you are consideringinstalling it, you should be aware of the followingWhat can be done to important points: minimise damage? the cost of purchasing, operating and maintaining an airconditioning plant is high. If such a financial commitment is possible,The potential for damage to collections from the seriously consider getting a system which noteffects of relative humidity and heat is greatest only regulates temperature but is capable ofwhen relative humidity and temperature fluctuate controlling the relative humidity as well;rapidly, or are extremely low or extremely high.if airconditioning is used to control theThe damage can be minimised by modifying the environment, it should operate continuously.conditions, if possible, and creating buffer zones For example, it is tempting to turn off thebetween your objects and the extreme orairconditioner because of the high operatingfluctuating conditions.costs. But the cyclic process of turning it on and off is likely to be more damaging toThere are many ways of controlling temperature collections than no air-conditioning at all;and relative humidity. Some methods are betterthan others and their advantages andairconditioning systems have a limited life.disadvantages will be discussed. They will operate at greatest efficiency for 10 to 15 years; andThe measures you use to improve your buildings airconditioning systems should be wellenvironment should be selected so that you can maintained, otherwise you could experiencemonitor their effects and, if necessary, modify them.fluctuations in the environment.HeatingModifying theIt is sometimes necessary to heat whole buildingsconditions in buildings or individual rooms. Generally, heating is used tomake people comfortable. This is an importantconsideration; but you should be aware also that Airconditioningraising the temperature affects the objects in thebuilding.Airconditioning is the most obvious, but notnecessarily the best, method of controlling Heating affects relative humidity. Remember:temperature and relative humidity. The methodinvolves taking aireither fresh air from the heating a building in an already dryoutside or recycled air from the insideandenvironment could be disastrous, because itchanging its temperature and moisture content. will lower the relative humidity;26 Humidity and Temperature 8. heating a building when there is anThe building as a buffer zoneadditional source of water will evaporatemore of the water. The relative humidity mayremain unchanged or it may alter, depending Dont despair if your building is not air-conditionedon the amount of water available and theand you cant afford airconditioning. A well-amount of heat applied. This may not be the maintained building of solid construction provides aeffect youre hoping to achieve; andvery reasonable environment for collections.if you want to raise the temperature withoutA building made of thick stone walls or cavity-lowering the relative humidity, you needbrick construction with high ceilings provides goodadditional water. Additional water sourcesinsulation against climatic changes. In hotcould include mechanical devices such asweather, these buildings take a few days to heathumidifiers or, more simply, dishes or traysof water left to evaporate. up; and then, as the outside temperature drops,they lose heat slowly. Fluctuations occur, but theyoccur gradually.Altering the relative humidityMake sure your building is well-maintained, so thatit provides the maximum possible seal against It is possible to vary the humidity withoutfluctuations in the outside environment. This is markedly changing the temperature. Relativeparticularly important if the building is made of humidity can be reduced using a dehumidifier.light building materials and is in a fairly extreme This is a remedial measure which adjusts a environment. dangerously wet environment.Clean out gutters, repair cracks in walls andceilings, and have leaky roofs checked. Thisimproves the stability of the temperature andhumidity inside. If you are considering upgrading the buildings,remember to insulate walls and ceilingsespecially if the building is constructed fromlight-weight or heat-conductive materials suchas fibro and corrugated iron. By so doing, it cannot only help to modify the internal environment,but also ensure better preservation of thecollection as well as making people feel morecomfortable inside.Dehumidifiers. Use the features of the buildingPhotograph courtesy of Artlab AustraliaImproved conditions for the storage and displayof collections can be achieved by choosing good Moisture can be introduced to the air using either storage and display sites within the building, a steam generator or an ultrasonic humidifier. and using the features of the building to modify These are remedial measures which adjust a conditions. dangerously dry environment.The most stable area of a non-airconditioned These machines should not be seen as permanent building is an internal room on the ground floor solutions to your environmental problems. They because it is buffered against climatic changes. can be costly to run, they need fairly constant attention and can be bulky and noisy.There are other areas one could choose. Basementsare acceptable as they provide a cool temperature.However, they are likely to be damp. On the otherhand, an attic would often not be insulated, butwould be dry.Humidity and Temperature 27 9. If there is an optimum choice, the most sensitive Local climates and microclimatesobjects should be displayed in an internal room, orat least against an internal wall in preference toIn the same way that you use the features of aan external one.building to modify the conditions within thebuilding, you can use doors, windows, blindsIn warm, humid conditions, such as in tropical areas:and awningsto modify conditions withinair flow and good ventilation are important if individual rooms. you want to minimise damage to collections;There are a number of other steps which can beuse oscillating fans, with doors open to taken to create and modify microclimates within improve air movement; andthe building. consider other ways of cooling the inside of Layers of storage the building. For example, install interior blinds on windows to limit the amount of heat coming into the building, install exteriorWhen storing important objects, give them their shutters or awnings, or put up shade-cloth. If own microclimate by providing layers of storage as possible, plant trees around the building, but this provides some protection against climatic not too close because this gives insects easyextremes and fluctuations, even when conditions in access to the building.the room or building are difficult to control.If you are building a museum or modifying anMicroclimates also exists within glazed frames,existing building in the tropics, remember that display cases and boxes.non-airconditioned buildings should havebreezeways, if possible. For more informationIn arid or temperate zones, when the temperature There is information about conservation framingis extreme outside the building, you can limit in the chapters on Textiles, Photographs andtemperature fluctuations inside your building by:Paper in Caring for Cultural Material 1 and 2. keeping doors and windows closed;A display case is a box with transparent sides intowhich an object is placed. The walls of the boxkeeping self-closing doors well oiled, so that isolate the object to some extent. Passive control they shut quickly and fully;systems can also be set up within the box.installing interior blinds on windows, to buffer against outside conditions; Silica gel is the most common material used tocontrol relative humidity in display cases. It caninstalling exterior shutters or awnings, orbe pre-conditioned to achieve a desired humidity putting up shade-cloth; andlevel; the quantity placed in the case depends onthe volume of the case.planting trees around the buildingbut not too close because this gives insects easy For more information access to the building.Silica gel is not particularly easy to use.Encourage people to leave wet umbrellas and coatsFor more information about its useat the front doorby providing umbrella basinsrefer to the section on the use of silica geland coat hooks. This will prevent the introduction later in this chapter.of excess moisture.Other materials, such as Nikka pellets and Artsorb,operate on the same principles as silica gel. Cheaper alternatives which can be used includeclean, cotton scraps and shredded, acid-free paper.These absorb excess moisture, but do little tocorrect a dry environment.28 Humidity and Temperature 10. A word of cautionabout travellingexhibitions and loansRemember that your collections may be subject toa changing environment within and outside amuseum. Of particular concern is if items are lentto other organisations, or are transported for otherreasons. They should be well packed and sealed, toensure that they are not subjected to unacceptablefluctuations in transit.These documents are well protected fromenvironmental fluctuations by layers of storageOn arrival at the destination of transported items,individual sleeves plus a storage box.the local climate within the crate should beallowed to gradually adjust to the conditions ofPhotograph courtesy of Artlab Australiathe new environment. The crates should remainunopened at the destination for a full 24 hours. Acid-free wrappers, interleaving, mounting and This should also be done on the return journey. framing when used individually or in combinations, create layers of protection from If the objects are travelling from one extreme to extremes of, and fluctuations in, relative another, for example, from a tropical to an arid humidity. They create small, isolatedclimate, it may be advisable to allow more than 24 microclimates in which the relative humidity hours for conditioning at each end. fluctuates slowly. CAUTION Avoid sealing objects in plastic in tropical conditionsit will not allow them to breathe,creating a risk of mould growth. The hand on this polychrome sculpture split after getting very damp and then drying out quickly. Mould has grown on the sculpture as well. The sculpture became very damp when it was shipped from Europe to Australia. Photograph courtesy of Artlab AustraliaThe layers of mounting and framing materials bufferthese items against environmental extremes andfluctuations.Photograph courtesy of Artlab AustraliaHumidity and Temperature 29 11. In an ideal world... For the safe-keeping of collections in museums,galleries and libraries, the temperature should bekept constantin the range 1822C.The levels of relative humidityRHrecommendedfor the safe-keeping of collections in museums, These recommended levels are ideal. However, ingalleries and libraries are:some areas of Australia it is extremely difficult tocome close to achieving these recommended levels. 50% 5%; It is not always practical to put all our efforts intoachieving these levels within a building, when this is a compromise which has been arrivedat by assessing the average requirement of an there are many others ways of providing protection.average collection in an average climate;CAUTION it is almost impossible to achieve a constant Were not in an ideal world, so it is importantrelative humidity of 50%; so a margin of 5 percent either side of this has been permitted,to remember that many objects becomethat is, between 45% and 55%;conditioned to their environmentseven though these may be extreme. the danger zones for relative humidity areover 65%, when mould grows and metalA lot of damage can be done by attemptingcorrosion is common; and below 35%, when to place an object, which is stable insome materials dry out and become brittle; an extreme climate, into an environment that in tropical areas, 60% 5% is more realistic. conforms to the recommended levels. Australias climatic zonesARIDAn arid climate is generally very dry. For example:Av. Min RHAv. Max RH Av. Min Temp Av. Max TempBroken Hill25% 75%12.1C 23.7CKalgoorlie 23% 74%11.5C 25.1CAlice Springs17% 65%16.3C 33C In arid areas, it is often very hot during the day and very cold at night. This wide fluctuation ismatched by wide fluctuations in relative humidity. Take Alice Springs for example: temperatures have been known to range from 42C to 20C in summer; and in winter from 18C to -5C; and relative humidity can range from 75%20%. 30 Humidity and Temperature 12. TEMPERATE A temperate climate is considered a moderate climate.For example: Av. Min RH Av. Max RHAv. Min Temp Av. Max Temp Launceston 44%90% 6.2C16.8C Adelaide 34%79% 12C 22.1C Ballarat 40%91% 7.3C17.4CHowever extreme and fluctuations can be experienced in temperate areas. Take Adelaide for example: temperatures have been known to range from 40C to 15C in a day; relative humidity can range from 100%30%; and when it is hot in Adelaide it is often quite dry.Temperate climates tend to have a greater range of temperatures than tropical climates and may include extreme climatic variations. TROPICAL Tropical climates occur north of the Tropic of Capricorn. They are characterised by heavy rainfall, high humidity and high temperatures.For example: Av. Min RH Av. Max RHAv. Min Temp Av. Max Temp Darwin 43%85% 23.8C 31.6C Townsville 51%75% 19.5C 28.6CAverage readings do not give a very good indication of the extremes that can be experienced. In Darwin, for example: temperatures can range from 35C to 20C in a day; relative humidity can range from 100%50%; and high temperature and relative humidity tend to coincide.Note: Townsville may not be considered tropicalit may be more accurately classified as sub-tropical. It must be remembered that these categories are only a guide. Climates change gradually and there are many areas in Australia that would be difficult to place in these very broad categories.Humidity and Temperature 31 13. MORE ABOUT RELATIVEHUMIDITY AND TEMPERATUREMeasuring relative humidityBeing able to measure relative humidity is notabsolutely necessary when controlling fluctuationsor extremes; but it is helpful in identifyingproblems. If the environment is very dry or very damp, youA whirling hygrometer or sling psychrometer.can generally feel ityour skin responds to these Photograph courtesy of Artlab Australiaconditions. We feel dry and sometimes a bit itchyin very dry conditions, and clammy in humidThe amount of water which evaporates depends onconditions. But feeling the extremes doesnt tell usthe capacity of the air to contain water. Thehow extreme the conditions are; and it gives us nodecrease in temperature shows how much waterindication of how rapidly the conditions arehas gone into the air; this shows the capacity offluctuating.the air to contain water.To gather information about the levels, you needThe dry bulb gives the temperature of the air.to measure the relative humidity and temperature;and to get information about the rate ofBy comparing the two temperatures after thefluctuation, you need to monitor the environment.instrument has been whirled, the relative humiditycan be calculated. This is made easier by usingMeasuring the conditions involves taking readingspublished tables listing the relative humidityat a specific timea snapshot of the conditions.against the dry and wet-bulb temperatures. Thesetables should be provided with the hygrometer.Monitoring conditions involves continuous orrepeated measurement, so that you get a changingThe advantages of using a whirling hygrometer are:picture or series of snapshots over a period of time. they are relatively cheap;There are a number of devices which can be usedto measure relative humidity. they dont require ongoing maintenance; and Whirling hygrometer if you purchase another measuring device,you will still need a whirling hygrometer, as areference device to calibrate the other device.A whirling hygrometeralso called a slingpsychrometermeasures relative humidity and The disadvantages of using a whirling hygrometertemperature directly. are: It has two matched thermometers. One thermometer they provide a snapshot only. They do notis called the dry bulb and the other the wet bulb.continuously monitor the environment;The wet bulb thermometer has a cotton sleevewrapped around its base. Distilled water from a to monitor the environment with a whirlinghygrometer, you need to take readings in thesmall reservoir is used to keep the sleeve wet.same places at regular intervals, and recordwhen and where you took the readings; andThe hygrometer is whirled around in the air. Whilethis happens, water from the sleeve of the wet whirling a manually-operated hygrometer canbulb thermometer evaporates, and the wet-bulb make your arm tired. Battery-operatedtemperature shown by the thermometer goes down. whirling hygrometers are available.32 Humidity and Temperature 14. Thermohygrograph the chart also shows when fluctuations occurred, so you can relate fluctuations to events in the area being monitored, for example, the arrival of A thermohygrograph allows for continuousa bus-load of tourists on a wet day! measurement of relative humidity and temperature over a period of time. It monitors theThe disadvantages of using a thermohygrograph are: environmental changes. they require ongoing maintenance, to ensure Thermohygrographs work on the principle thatthey are recording accurately and to ensure organic materials expand and contract as thethat the cylinder is rotating at the right rate; relative humidity changes. In this case, the organic material is human hair. they need to be calibrated periodically, and the hairs need to be re-conditioned; and The hairs are bundled together and stretched thermohygrographs give you information only between a fixed pin and a moveable pin. The if you look at the charts. Most people look at moveable pin is attached to a series of levers, the charts only at the end of the recording which amplify the movement of the hairs.period, so they dont respond immediately to problems as they arise. A pen is attached to the end lever; and this pen plots the movement of the lever on a chart, which Dial hygrometers is mounted on a rotating cylinder. The rate of rotation can be altered, so that the relative humidity is plotted over a day, a week or a month.Dial hygrometers work on the same principle as the thermohygrographusing human hair to operate a A thermohygrograph also has a temperature lever which moves a dial. sensorwhich records the temperature on the same chart.The chart should be changed at the end of each recording period. When you change the chart, set the pen on the correct time, and you will have a record of the times when changes occur.The advantages of using a thermohygrograph are:the thermohygrograph chart containsinformation about temperature and humidity, aswell as the relationship between the two; andA dial hygrometer. Photograph courtesy of Artlab Australia The advantages of using a dial hygrometer are that it is small and can be placed in display cases and on shelves.The disadvantages of using a dial hygrometer are: they measure relative humidity, but not the temperature; and they measure continuously, but dont recordA thermohygrograph.the informationyou have to look at themPhotograph courtesy of Artlab Australiacontinuously if you want to use them to monitor changes.Humidity and Temperature 33 15. Electronic hygrometers We will not give detailed instructions for the calibration of individual instruments because there will be slight variations, depending on the type ofElectronic hygrometers are generally used to instrument you havewhether a thermohygrographprovide a snapshot of conditions. They measure or a dial hygrometer. The instrument will comerelative humidity and temperature, and need to be with instructions. If it does not, ask the suppliercalibrated periodically. Before they are used, they for clear instructions. If you dont follow theneed to be allowed to acclimatise to the area they instructions, your readings wont be accurate andwill be monitoring. cant be relied on. Data loggers If you buy a thermohygrograph, you will need to buy a whirling hygrometer as well. If funds areSystems which monitor relative humidity andlimited, the whirling hygrometer would be a wisertemperature, and download data to computers areinvestment.now availablethey are called data loggers. The advantages of using data loggers are:Dehumidifiersthey can be linked to alarms so that when conditions move outside the recommended A dehumidifier is basically a cooling coil levels, action can be taken; andairconditioner. Instead of conducting the compressor heat out of the building, the heat isthe remote sensors can be placed in display cases, storage boxes and crates.retained inside the buildingand so the temperature does not change, except when theThe disadvantages of using data loggers are: dehumidifier is in a small room. In this case, the temperature in the room can be raised by theyou need a computer to access the operation of the dehumidifier. information; and Moisture from the air, however, is still condensedthey are expensivealthough they are likely on the cooling coils, and taken away by a hose or to become cheaper as time goes by.collected in a bucket. Dehumidifiers are a remedial measure to adjust a dangerously wet environment. Humidity indicator cards If using a dehumidifier to dry an area of your building, remember to empty the catchmentHumidity indicator cards are also available. These bucketthe buckets are not very big.use moisture-sensitive salts which change colouras the relative humidity alters. They can be veryuseful for low-cost monitoringespecially withindisplay cases and storage boxesprovided youcheck them regularly.Humidifiers andsteam generatorsSeparate temperature cards are needed if you wantto check temperature variations. Moisture can be introduced into the air by using either a steam generator or an ultrasonic Calibration humidifier. Thermohygrographs and dial and electronicA steam generator uses heat to create steam.hygrometers do not remain accurate. Ideally, theyThe steam is then cooled to form a water-vapour,should be calibrated against an instrument such as a which can be introduced into the museum.whirling hygrometer once a month and if they haveslipped out of calibration, they should be An ultrasonic humidifier uses a small crystalrecalibrated. The hairs in thermohygrographs and vibrating at very high frequencyto smash liquid-dial hygrometers must also be reconditionedwater into tiny droplets. The droplets are smallregularly. enough to be suspended in air as a cold vapour.34 Humidity and Temperature 16. Both devices are remedial measures to adjust aSilica gel also needs to be reconditioned, because dangerously dry environment.it absorbs water and retains it. The silica gel has to be removed from the case and reconditioned NB. The water in humidifiers and steam generators usually by heating in an ovenand then put back needs to be topped up regularly. If this is not in the case. You may need two batches of silica done, the steam generator or ultrasonic humidifiergel, so that when you are reconditioning one, the could be severely damaged.other is in the case.Display cases that are to contain silica gel should The use of silica gel ideally have separate compartments: one for the object and one for the silica gel; and there should be air flow between these compartments. Individual display cases can act as buffer zones, and maintain humidity at reasonably constantCAUTION levelsprovided the temperature does not vary greatly. However, fluctuations can occur and it isIf you use silica gel in its sometimes necessary to use buffering materials: granulated form, it is advisable to silica gel, for example.wear a dust mask.Silica gel is often seen as a simple solution to environmental problems. In some museums and If you have a problem relating to temperature galleries, small bags of silica gel are placed in and humidity and how to manage it correctly display cases and left there permanently. for the preservation of your important objects, Unfortunately, this has almost no effect. contact a conservator. Conservators can offer advice and practical solutions. Using silica gel is not simple. The calculation used to determine the amount of silica gel required is complex. It involves a knowledge of the daily rate of air-changes in the case, the local humidity For further reading conditions and the volume of the case. Stolow, Nathan, 1987, Conservation and The amount of silica gel required is far more thanExhibitions: Packing, Transport, Storage and most people imagine. The amount required canEnvironmental Considerations, Butterworth and range from approximately 7kg/m3 to about 20kg/m3,Co., London. depending on conditions. This is a lot of silica gel. Thomson, Garry, 1994, The Museum Environment,3rd edn, Butterworth-Heinemann, Oxford.Self-evaluation quiz Question 1.Which of the following statements are false?a) Extremes of temperature and relative humiditycan cause damage to objects in museums,The dish of blue silica gel is ready for use. It will galleries and libraries.absorb moisture and so remove water from theatmosphere. When silica gel has absorbed all the water b) Most insects and mould thrive in warmerit can, it turns pink. When the silica gel is pink, itconditions.needs to be reconditioned.Photograph courtesy of Artlab Australiac) Relative humidity and temperature are closelyrelated.Humidity and Temperature 35 17. d) Relative humidity can affect the comfort of c) the amount of water-vapour in the air and people in museums, galleries and libraries,the total amount of water-vapour that the air but wont affect the collections.can hold at a particular temperature;Question 2. d) the humidity inside relative to the humidityoutside. In high relative humidity conditions: Question 6.a) bark paintings dry out and crack; Which of the following statements are true?b) dyes and textiles fade and deteriorate quickly; a) Changes in temperature can often lead toc) moulds become too wet to grow; significant alterations in relative humidity. d) mounted vellums become taut.b) People have no effect on relative humidityand temperature levels in museums, galleries Question 3.and libraries.c) If the temperature inside a sealed box drops,Rapid fluctuations of relative humidity:the relative humidity inside the box will beraised.a) subject materials to constant movement as they absorb moisture and give it out again; d) Display cases have 0% relative humidity. b) can cause extreme damage;Question 7.c) can alter the chemical composition of some minerals; The climates which are relevant to objects in museums, galleries and libraries are:d) should be avoided; a) microclimates;e) all of the above. b) the climates within their storage and display Question 4.areas;c) the regional climate;If a collection has become conditioned to anextreme environment, you should: d) the climate in the building in which they arestored;a) alter the environment to meet the recommended ideal conditions because this e) all of the above. will be better for the collection;Question 8.b) concentrate your efforts on maintaining a stable environment; In warm, humid conditions:c) send the collection to a more moderate climate; a) good ventilation and air flow help to preventd) none of the above. mould outbreaks;Question 5. b) you should shut all doors and windows toprevent mould spores entering the building; Relative humidity is a comparison between: c) dehumidifiers should be used to dry the air; a) water in the air and temperature; d) seal your objects in plastic. b) the amount of water-vapour in the air at different temperatures;36 Humidity and Temperature 18. Question 9. Question 5.To protect important objects from fluctuations in Answer: c). relative humidity and temperature, you should:Question 6. a) provide them with layers of storage;b) ensure the building is well maintained;Answer: a) and c) are true.c) get air-conditioning installed; Question 7. d) buy a steam generator. Answer: e).Question 10.Question 8. Layers of storage to protect against fluctuations and extremes of relative humidity can be created by:Answer: a) is correct. b), c) and d) are not correct. It is no use shutting doors and windows to keep a) placing items in storage boxes;mould spores out because they are everywhere anyway. Dehumidifiers should be used only as a b) interleaving or wrapping objects;remedial measure to adjust a dangerously wet c) placing items in display cases for exhibition; environment. In warm, humid conditions, you should avoid sealing objects in plastic, because it d) mounting and framing;will not allow them to breathe, creating a risk of mould growth. e) all of the above;Question 9. f) combinations of the above.Answer: a) and b). You could install airconditioning; but its not completely necessary Answers toif you are able to create suitable microclimates.self-evaluation quiz Steam generators should only ever be used to adjust a dangerously dry environment. Question 1. Question 10.Answer: d) is false.Answer: e) and f). The methods used will depend on the object type.Question 2.Answer: b) is correct. a), c) and d) are incorrect. Bark paintings and vellums absorb moisture. This can cause changes in shape and distortion. Mounted vellums sag. Moulds thrive. Question 3.Answer: e). Question 4.Answer: b). The emphasis should be on stability.Humidity and Temperature 37