Pigment Questions pigment and dyes Pigments dyes ?· Pigments and dyes are similar in that they impart…

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  • Pigment Questions

    1. Know the meaning of the terms pigment and dyes. How are they similar? How are they different?

    Pigments and dyes are similar in that they impart color to a medium. Pigments are suspended in a binder (such as linseed oil). While pigment and binders works well on wood, stone, ceramics, and mounted fabrics, it works poorly on flexible materials that were meant to be worn. To color such fabrics one needs dyes. Dyes work into the fabrics and retain the flexibility of the fabric.

    2. Know the color and chemical structure for the pigment verdigris.

    Verdigris is a green pigment copper (II) acetate.

    3. What is the oxidation state of copper in copper acetate?

    Copper (II) = Cu2+

    4. What ingredients did you use to make verdigris?

    Copper metal and vinegar.

    5. What is vinegar, chemically? What is its chemical structure?

    A dilute (5%) aqueous solution of acetic acid, CH3CO2H

    H3C CO


    6. Where does vinegar come from?

    Vinegar comes from wine that has gone bad. If wine is left exposed in a non-sterile environment, bacteria can get in. These bacteria metabolize the ethanol to ethanoic acid, a.k.a, acetic acid. The conversion of ethanol to acetic acid is an oxidation.

    7. What is an acid?

    An acid is a substance that gives up (or donates) a proton, H+.

    8. Write the chemical equation that shows how acetic acid gives a proton.

  • O



    O+ H

    acetic acid



    8. What is the pH? How is it defined?

    pH is a measure of the concentration of the hydronium ion, [H3O+]. The definition of pH is pH = log [H3O+]

    9. What is the pH of an acetic acid solution? Less than 7, equal to 7, or greater than 7?

    pH < 7

    10. What chemical reaction takes place when copper reacts with acetic acid?

    Cu + 2 CH3CO2H Cu (CH3CO2)2+ H2

    11. What has happened to the copper in this reaction?

    The copper has been oxidized. It has lost two electrons.

    12. What happened to those electrons?

    They are used to reduce the H+ to hydrogen atoms:

    Cu + H Cu + H

    Cu + H Cu + H2++

    Cu + H +2 Cu 2+ H2 (gas)

    Notice that the two hydrogen atoms (H) combine to form hydrogen gas.

    13. What type of reaction is this?

    This is a redox (reduction-oxidation) reaction. The copper is oxidized and the acetic acid reduced. Note that reductionoxidation reactions always occurs in pairs: one species is reduced and one is oxidized.

    14. The verdigris you made was initially a 1:1 mixture of copper acetate and copper hydroxide. How did you make this in to pure copper acetate?

    Recrystallization from vinegar.

  • Dye Questions

    1. Know the meaning of the terms pigment and dyes. How are they similar? How are they different?

    Pigments and dyes are similar in that they impart color to a medium. Pigments are suspended in a binder (such as linseed oil). While pigment and binders works well on wood, stone, ceramics, and mounted fabrics, it works poorly on flexible materials that were meant to be worn. To color such fabrics one needs dyes. Dyes work into the fabrics and retain the flexibility of the fabric.

    2. Know the meaning of the terms fugitive and colorfast.

    A fugitive dye is one that doesnt hold its color after washing. A colorfast dye is one that remains even after washing.

    3. Why must wool be washed before it can be dyed?

    Wool contains a natural grease called lanolin. Washing with soap removes the lanolin that would otherwise interfere with the ability of the dye to adhere to the fiber.

    4. What is soap? Know its chemical structure.

    Soap is a particular molecule (aliphatic carboxylic acid) that has both a polar, water soluble component (the carboxylic acid) and a greasy, non-polar component. Normally, water and grease dont dissolve in each other and so water cant remove grease. Soap dissolves both grease and water, so that water-soap mixtures can remove grease and oil.




    ionic endwater attracting

    hydrocarbon endwater repeling

    5. Know the meaning of the terms direct dyeing, mordant dyeing, and vat dyeing. What are the similarities and what are the differences?

    Direct, mordant, and vat dyeing are three different ways for coloring fabrics. In direct dyeing the dyestuff is applied directly to the fabric. In mordant dyeing a mineral is used to help the dye bind to the fabric. In vat dyeing the dye is chemically modified to make it water soluble.

  • 6. Give an example of each of the dyeing methods that you carried out in the laboratory.

    Direct: dyeing with walnut shells Mordant: dyeing with cochineal Vat: dyeing with indigo

    7. What is the main chemical responsible for the color that comes from walnut dye? Know its name, chemical structure, molecular formula, and its color.

    The main colored material in the walnut dye is juglone. It makes a brown dye on exposure to air.




    = CC










    Note that the skeletal structure on the left is the normal way we would written it, but it contains hidden information: carbons at the vertices and hydrogens added until each carbon is tetravalent. From is structural drawing we can see that it has 10 Cs, 6 Hs, and 3 Os = C10H6O3.

    8. What is the main chemical responsible for the cochineal dye? Know its name, chemical structure, molecular formula, and its color.

    The main colored material in the cochineal is carminic acid. It is a deep red dye.












    Its molecular formula is C22H20O13.

    8. Know the meaning of the term mordant. Give some examples of mordants.

    Many dyes can be made colorfast by the use of certain minerals called mordants. Potash and soda ash can in some cases serve as mordants, but they are not commonly used because they are basic and thus tend to harm the fabric. Much milder and far more common is the aluminum mineral alum.

    9. Know the chemical names and formulae for potash, soda ash, and alum.

    potash potassium carbonate K2CO3 soda ash sodium carbonate Na2CO3

  • alum potassium aluminum sulfate KAl(SO4)2

    10. Know the common oxidation state of potassium, sodium, aluminum, carbonate, and sulfate.

    For simple monoatomic atoms like potassium, sodium, and aluminum the common oxidation states can often be determined from the atoms position on the periodic table. For polyatomic species like carbonate, and sulfate the structures and oxidation states needs to be dedicated to memory.

    potassium + 1 K+ sodium + 1 Na+ aluminum + 3 Al3+ carbonate 2 CO32 sulfate 2 SO42

    11. Know how to draw Lewis structures for carbonate and sulfate.


    O O

    C 1 x 4 e = 4 O 3 x 6 e = 18+ dianion 2 e = 2 valence e 22

    O SO


    S 1 x 6 e = 6 O 4 x 6 e = 24+ dianion 2 e = 2 valence e 32

    12. Know what alum is and how it works.

    Alum is water soluble aluminum salt potassium aluminum sulfate that functions as a mordant to help a dye bind to fabric. The important part of alum is the Al3+. The K+ and SO42 are spectator ions. They are not directly involved in the chemical binding, but they do help make the aluminum water soluble.

    Aluminum binds strongly to oxygen. The mordanting process involves two steps. During the first step in which the fabric is soaked in an aqueous solution of alum, the aluminum binds to the fabric. During the second step in which the mordanted fabric is soaked in a dye solution, the dye binds to the aluminum.





    O O O O

    mordanted fiber

    Al Al

    OH OH



    O O O OAl Al

    O Odye dye


  • 13. What is indigo? Know its name, chemical structure, molecular formula, color, and where is comes from.

    Indigo is a blue dye made from the plant Isatis tinctoria or Indigofera tinctoria. It can also be made synthetically in the laboratory. Based on the structure below its MF is C16H10N2O2.



    H O


    14. Describe how vat dyeing of indigo works.

    Indigo and related compounds such as Tyrian purple are highly insoluble in water and most other solvents. Lacking the ability to dissolve in water, indigo can not function as a dye. However, when indigo is placed in a reducing solution such as dithionite (Na2S2O4), it is reduced to leucoindigo. In the reduction process, the indigo gains an extra electron and is therefore an anion that makes it water soluble. Now that it is soluble it can move wherever the water can go, i.e. into the fibers. The leucoindigo lacks color and stays colorless as long as is remains in the reducing solution. However, leucoindigo is not stable and when the fabric is removed from the reducing conditions it is exposed to oxygen in the atmosphere and spontaneously reverts to the blue indigo. The whole process is known as vat dyeing. The chemistry can be summarized in the following chemical reactions:



    H O




    H O




    H O





    Glass Questions

    1. What is the main ingredient in glass?

    Silicon dioxide, SiO2.

    2. What is the three dimensional structure of silicon dioxide?

    Each silicon atom is bound to four oxygens and each oxygen is bound to two silicon atoms, i.e. the silicon is tetravalent and the oxygen is divalent. Both kinds of atoms are essentially tetrahedral but in the case of oxygen there are two lone-pairs of electrons.

  • 3. What is the difference between calcite (SiO2) and obsidian (also SiO2)?

    In calcite the atoms are aligned in a regular, orderly, repeating pattern and is said to be crystalline. In obsidian the silicon oxide units are arranged in a random, disordered pattern and is said to be amorphous. This amorphous solid is also called a glass.

    4. Under what conditions are crystalline materials made? Glassy materials?

    Crystals form when liquid or molten materials cool slowly. Glasses form when liquid or molten materials quickly.

    5. Why is pure silica not often used in glassmaking?

    Its melting point is too high (1710C, 3110F).

    6. What is the chemical composition of soda-lime glass?

    Soda-lime glass consists of 1020% soda (NaO), 510% lime (CaO), and 6070% silica (SiO2).

    7. What is the function of the soda in soda-lime glass?

    Soda acts as a flux to lower the melting point of the silica.

    8. What is this function of the lime in soda-lime glass?

    The sodium in glass actually weakens the glass because sodium cations are highly water soluble. The calcium stabilizes the glass because calcium cations have poor solubility in water.

    9. Soda and lime are generally too unstable to be found pure in nature. What are the actual sources of soda and lime?

    Both soda and lime are made by calcining (roasting) their carbonates during the glassmaking process:

    CaCO3 CaO Na2CO3 Na2O



    sodiumcarbonate"soda ash"


    10. Soda-lime glass is the most common type of glass in ancient and modern times. The glass you made in the lab however is a more recent invention, a borosilicate glass. What ingredients go into borosilicate glass?

  • Borax, B4O5(OH)42, and silica, SiO2.

    11. What is the purpose of the borax in the borosilicate glass?

    Borax is the flux that lowers the melting point of the mixture.

    Lost Wax Questions1. What is wax?

    There are many types of wax (beeswax, paraffin wax, etc.) and almost are mixtures of waxy compounds. The common feature of all waxes is a long hydrocarbon chain. They are non-polar and repel water.

    2. How does the lost wax process work?

    A model is first carved in wax. The wax model is invested in plaster (see below) or clay. The wax is burned away when the plaster or clay is fired to leave a mold. Molten glass or bronze can then be cast into the mold.

    3. What is the chemical reaction that takes place when the wax is lost? Balance the reaction.

    For a paraffin wax with 25 carbons,

    C25H52 + 38 O2

    25 CO2 + 26 H2O

    Glaze Questions1. What is glaze?

    Glaze is a glass like material that binds to the surface of ceramic vessels. 2. What is the chemical composition of glaze?

    There are thousands of different recipes for glazes, but they all have the same basic ingredients: (a) silica (SiO2), (b) a flux, such as soda (sodium oxide, Na2O), galena (lead sulfide, PbS), or cassiterite (tin oxide, SnO), and (c) a colorant such as malachite (copper carbonate, Cu(CO3)).

    3. When is a ceramic glazed?

    The glaze is usually an external coating either painted or dipped onto a ceramic that has already been fired. Thus, a second firing is required to bake the glaze onto the ceramic.

  • 4. What is Egyptian paste?

    Egyptian paste is a self-glazing clay-like material. It is plastic (moldable) when wet, but dries firm. The glaze is not coated coated on the outside as described above. Rather the glaze ingredients (silica and flux) are mixed into the wet clay. As the water evaporates from the clay, the water and flux (soda ash) are drawn to the surface. When fired the glaze forms only at the surface, where both silica and flux are found.

    Plaster Questions

    1. What is plaster?

    Plaster is an ambiguous term with at least two different meanings. It can refer to either lime plaster or plaster of Paris. The two plasters have chemically distinct compositions.

    2. What is plaster of Paris?

    Plaster of Paris is calcium sulfate hemihydrate, CaSO4 H2O.

    3. What is the oxidation state of calcium in plaster of Paris?

    Calcium (II) = Ca2+

    4. Know how to draw Lewis structure for sulfate.

    O SO


    S 1 x 6 e = 6 O 4 x 6 e = 24+ dianion 2 e = 2 valence e 32

    5. What is the H2O in CaSO4 H2O?

    The dot water refers to the waters of hydration, or the amount of water in the crystal. In this case there is half a water molecule for every molecule of calcium sulfate. It makes more sense to think of this as two molecules of calcium sulfate sharing one water molecule in its crystal.

    6. How is plaster of Paris made?

  • Plaster of Paris is made from by heating a mineral called gypsum, CaSO4 2 H2O, to about 150C Some, but not all, of the water is driven off:

    CaSO4 2 H2O150C

    CaSO4 1/2 H2O

    (At higher temperatures it is possible to drive off all the water and create anhydrous calcium sulfate, but this material is not as useful as a plaster ingredient.)

    (A large deposit of gypsum is found in Montmartre in Paris, hence the our name plaster of Paris.)

    7. How is plaster of Paris used?

    Plaster of Paris is a dry powder. After water is added, it liberates heat and becomes a moldable (plastic) material that hardens slowly into gypsum, which can be further worked by carving or sanding because it is a relatively soft material. In effect this completes the cycle of gypsum to plaster of Paris to gypsum:

    CaSO4 2 H2O150C

    CaSO4 1/2 H2OH2O

    CaSO4 2 H2O

    gypsumcalcium sulfate


    plaster of Pariscalcium sulfatehemihydrate

    gypsumcalcium sulfate


    Typically, plaster of Paris is mixed with various amounts of silica (SiO2) to give it more strength.

    8. What is lime plaster?

    Lime plaster is slaked lime with silica added to give it strength. The chemical formula for slaked lime is Ca(OH)2, also known as calcium hydroxide.

    9. Where does lime plaster come from?

    Slaked lime, or lime plaster, is made from a mineral called limestone, which chemist call calcium carbonate, CaCO3.

    10. Know the Lewis structure for carbonate.

  • CO

    O O

    C 1 x 4 e = 4 O 3 x 6 e = 18+ dianion 2 e = 2 valence e 22

    11. How is the limestone processed to give lime plaster?

    Limestone is first calcined (roasted) at 850C to drive off CO2 giving lime (sometimes called quick lime), which chemists call calcium oxide.





    + CO2

    The calcium oxide is unstable in the presence of moisture. It reacts with water to give hydrated, or slaked, lime:



    "quick lime"

    H2OCa(OH)2 ( + heat )


    "slaked lime"

    12. How does slaked lime react with the air?

    Slaked lime is unstable in air and slowly reacts with the carbon dioxide in the air to form calcium carbonate,


    hydroxide"slaked lime"



    thus completing the cycle from limestone to quick lime to slaked lime back to limestone:




    CaCO3"limestone" "limestone""quick lime" "slaked lime"

    13. What is the point of converting limestone to limestone?

    The slaked lime plaster is a plastic material, one that can be molded and shaped. Then, with time, it slowly converts to a hard, durable mineral.

    14. How is slaked lime used?

  • Slaked lime has many uses. When mixed with water it forms a spreadable plaster that can be used in art, architecture, and building. Slaked lime is almost always admixed with a variety of materials to make it stronger. When mixed with course sand, one gets mortar, a strong cement used by bricklayers. When mixed with finely ground sand (called flint by potters), one gets the material used to plaster a wall, which can be adorned with a fresco painting.

    India Ink Questions

    1. What is India ink? What color is it?

    India ink is a black, aqueous suspension of soot. It is also known as lamp black.

    2. How is soot made?

    Soot is the product of incomplete combustion. Most organic materials burn completely in oxygen to give carbon dioxide and water (and nitrogen oxides, if nitrogen is present).

    However, if the burning material does not mix thoroughly with oxygen, the carbon is not oxidized all the way to carbon dioxide, but rather to fine, black, graphitelike material (mostly carbon).

    3. What is gum arabic and what is its function in India ink?

    Gum arabic is a starchy material that comes from the Acacia tree in Ethiopia. It dissolves readily in water. Soot particles, which are not water soluble, are suspended in gum arabic. Thus, the function of the gum arabic is to bring the black pigment into solution.

    Paper/Papyrus Questions

    1. Why does water have such a high boiling point compared to molecules of similar weight?

    The attractive forces between molecules of, say, methane (MW = 16 g/mole) are very weak compared to the attractive forces in water (MW=18). It is easy to over the attractive forces between methane molecules, which happens at its boiling point of 162 C, but much are harder to overcome in water, which has a boiling point of +100C.

    2. What intermolecular force is holding the water molecules together?

    Hydrogen bonding.3. What is hydrogen bonding?

  • Hydrogen bonding is a special intermolecular force between electronegative atoms and hydrogens atoms on electronegative atoms, for example, between lone-pair on O and the HO bond. It is often drawn as a dipoledipole attraction:


    HH O



    4. How strong are hydrogen bonds?

    Relatively weak, about 1-5 kcal/mol. Covalent bonds are typically much stronger, 80-100 kcal/mol. However, many hydrogen bonds taken together can be quite strong.

    5. What is the primary chemical component of paper and papyrus?

    The main chemical component in paper and papyrus in cellulose.

    6. What is cellulose?

    Cellulose is a strong rigid polymer made from monomeric glucose units:

    7. What holds the strands of cellulose together in paper and papyrus?

    A network of hydrogen bonds.


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