SILICONE SOLVENTS IN PAPER CONSERVATION: BENCHTOP EXPERIMENTS

Austin Plann Curley ARTC 665

Richard Wolbers

Abstract Octamethylcyclotetrasiloxane is a cyclic silicone compound that has been recently

introduced to the fields of textile and paintings conservation. This paper reports on

benchtop experiments with this solvent, commonly referred to as D4, examining its

effects on water sensitive surfaces and considering its potential as a masking agent during

local aqueous treatment of paper objects.

Introduction Conservators treating paper-based material are keenly aware of the threat local

application of moisture poses for leaving tidelines in paper substrates. The most

commonly used adhesives in paper conservation are water-based, and their use on dry

paper objects is an unavoidable reality. Tear mending is a frequently performed

procedure that calls for application of a water-based adhesive to a dry paper substrate.

When performing tear mends to water sensitive materials, conservators typically work

with a thin, dry wheat starch paste or, a less hydrophilic alternative, cellulose ethers.

Additionally, conservators have used a technique of introducing volatile non-water-

miscible solvents to tear edges before applying adhesives, the idea being that the solvent

blocks lateral movement of water before evaporating away.

Tidelines occur when introduced moisture moves laterally through the paper fabric

carrying visible water sensitive materials with it. These materials, which may include

dirt, design media, degraded size, short-chain cellulose breakdown products, as well as

any number of fillers and other polar additives to paper, concentrate at the wet/dry

interface and remain there as moisture evaporates, leaving a halo of discoloration around

a lighter center. In addition to depositing materials at the wet/dry interface, the actual

event of the interface may itself initiate a degradation process in cellulose that leads to

oxidative reactions that generate colored products at the site of the interface.1

This experiment compares the ability of three solvents to protect water sensitive surfaces

during local aqueous treatments. Special attention is paid to octamethylcyclotetra-

siloxane, a cyclic silicone solvent recently introduced to the fields of textile and paintings

conservation by Richard Wolbers.

D4 - Technical Information D4 is the common name for octamethylcyclotetrasiloxane, a cyclic compound composed

of a ring of four alternating silicone and oxygen atoms with methyl groups branching

from each silicone. D4 belongs to a class of compounds known as cyclic siloxanes, or

cyclosiloxanes. At room temperature D4 is a clear, odorless and tasteless liquid. The

substance is moderately volatile and poorly soluble in water. D4 is used most frequently

in industrial settings, but it is a common ingredient in domestic personal care products

like antiperspirants and deodorants, skin creams and lotions, shaving products and make-

up. In the cosmetic industry D4 and other cyclic siloxanes may appear on ingredient lists

as simply “cyclomethicone”. The compound is also used in household care products such

                                                                                                               1 This process is initiated as differential swelling of the cellulose fibers creates mechanical stresses that rupture the cellulose chain, leaving radical products at each terminus. Paul M. Whitmore explains this phenomenon in his chapter on Paper Aging and the Influence of Water in Paper and Water.  

as polishes and wax blends where it functions as a solvent for larger molecules. The

concentration of D4 in personal cosmetic products is typically less than 10% by weight.

In household care products, D4 may account for up to 50% of the composition, but

numbers in the 1-5% range are more common. The other available cyclic siloxanes are

D3, D5, and D6; in each case the name refers to the number of silicone-oxygen bonds in

the ring.

C8H24O4Si4 octamethylcyclotetrasiloxane Commonly known as D4

Table 1: Physical Properties of D4 PropertyP Property Value

Physical state Liquid Color Clear Odor Odorless Molecular weight 296.61 Melting/Boiling point 17.7°C/175°C Density 0.95g/cm3 at 25°C Vapor pressure 135 Pa at 25°C Flammability Yes Flash point 51-61°C

Human Safety Minimal health risks are associated with exposure to D4 in small amounts. OSHA warns

of mild skin and eye irritation as well as liver damage and reproductive damage with

chronic exposure. The substance may be absorbed through the skin or by inhalation.

Workers in locations where D4 is made or used are required to wear personal protective

equipment to shield the eyes and skin. No special handling measures are required of

professionals using products containing D4. Likewise, consumers of domestic personal

care products are not required to take special precautions. The website of Sigma Aldrich

provides a bibliography with links to peer review papers concerning D4, many of which

examine potential health threats.

Environmental Safety

D4 is released into the air and wastewater with industrial use and with disposal of

consumer products. Wastewater treatment is able to removes most D4 from wastewater.

Residual amounts degrade, evaporate into the air, or bind to solids, which are deposited

in aquatic sediment. Testing of environmental persistence and bioaccumulative potential

is ongoing. The substance has been detected in aquatic sediment, though not at levels that

pose significant risk to plants or animals.

Prior Use in Conservation Previous conservation applications of cyclosiloxanes include use as a cleaning agent for

soiled textiles, and as a mask for water sensitive media during aqueous cleaning of a

mural on Masonite®.

Textiles Frances Lennard, a textile conservator at the University of Glasgow, presented at the

American Institute for Conservations annual meeting in 2013 on the use of D5 as a

“green” cleaning solvent for textiles. She used Fourier Transform Infra Red spectroscopy

to search for changes in molecular composition of aged and unaged textile samples after

immersion in D5. She concluded that there was no measurable change in immersed

samples and that the samples had the same feel and appearance soon after drying.

Paintings Students and interns of Joyce Hill Stoner are currently cleaning a 1939 mural Industrial

Claymont by Water Pyle Jr. using a treatment developed by Richard Wolbers that uses

D4 as both a masking agent to water sensitive media, and as an “oil” component in a

microemulsion. D4 is swabbed onto a small (~2”x2”) area of the surface, saturating it

thoroughly. With the solvent in place, pores are effectively sealed, allowing cleaning to

procede at the surface without disturbing water-sensitive media. The microemulsion2 is

applied and removed with a brush before the solvent evaporates.

Experimental Tests were designed to quickly examine the possibility of D4 as a stamping out agent

during aqueous treatment of works on paper. For comparison, D4 was tested alongside

ethanol and benzyl alcohol

Samples were prepared from laid sheets of gelatin-sized linen/cotton paper made by

Ruscombe Mills in France3. To make a water-sensitive surface, the paper was dipped in

McCormick Red Food Color4 and allowed to air dry. The result was a uniformly colored

surface that was easily disrupted by water with noticeable visual results.

Gross application The first test was intended to answer two questions: Does the solvent evaporate

completely? And does it do so without disrupting the media layer? To answer these

questions each of the solvents was grossly applied to the samples in droplets delivered

through a pipette and allowed to evaporate overnight.

Stamping out - water The second test examined the ability of each solvent to “hold in place” design media

during exposure to water. The test was performed in two steps: First a small area of the

                                                                                                               2  3:3:2 D4, L3 surfactant (laureth-3 polyethylene glycol), and water, with DTPA (diethylene triamine pentaacetic acid) as a chelator  3  The  paper  used  was  Ruscombe Mills Period End Paper 1790, a paper made for Talas and marketed as suitable replacement for endpapers of books dating from the 1880s through the first half of the 20th century.  4  McCormick Red Food Color is a mixture ot two dyes: FD&C Red 40 aka Allura Red, or Food Red (an azo dye, max absorbancy ~504 nm) and FD&C Red 3 Erythrosine (an azo dye, max absorbancy ~530 nm).  

sample was saturated with each of the three solvents, which were again delivered in drops

through a pipette. Next a small paintbrush was loaded with water and dragged through

the solvent saturated portion of the sample.

Stamping out - aqueous adhesives The last test asked the truly practical question of which if any of the solvents function

effectively as a mask during tear mends using aqueous adhesives. Evaluations were made

on a basis of each solvent’s ability to prevent tidelines during tear mends to scarf tears.

Adhesives used were wheat starch paste and methylcellulose. Solvents were wicked into

the tear edges by loading a Winsor and Newton size 0 brush and touching it lightly to the

paper fibers at the tear edge. The working time for each of the solvents is presented

below.

Table 2: Working Time of Tested Solvents Solvent Working time Ethanol 15-30 seconds Benzyl Alcohol 20 minutes or longer D-4 30-60 seconds Mends were otherwise made in the usual way; minimal amounts of adhesive were applied

by brush and the samples were dried under light weight between sheets of blotter with

interleaving layers of spun-bonded polyester. For comparison, additional mends were

made using the adhesives alone without pre-treating the tear edges with solvents.

Results/Discussion Gross application Of the three solvents tested, D4 was the only one that appeared to evaporate fully without

disrupting the appearance of the dyed surface. Ethanol left a dark tideline. Benzyl alcohol

did not appear to evaporate for several days and its odor remained for more than a week.

After evaporating fully, the solvent left the surface lighter in color, but with a diffuse

transition from the lightened to the unaffected area of the sample.

Image 1: Grossly applied solvents; control (none), ethanol, benzyl alcohol, D4

Stamping out - water None of the solvents performed well in ability to prevent applied water from disturbing

the dyed surface. Ethanol and benzyl alcohol in fact intensified the visual effect of water,

while D4 had little effect.

Image 2: Grossly applied solvents followed water applied directly by brush; control (water alone with no solvent), ethanol, benzyl alcohol, D4

Stamping out - aqueous adhesives The most successful tear mends, in terms of maintaining an undisturbed media layer,

were made with wheat starch paste alone, without preparing the tear edges with solvents.

These mends appeared lighter at the joined edges, but did not cause tidelines or

lightening beyond the join.

Image 3: Tear mends using wheat starch paste with solvent masks: control (wheat starch paste alone with no solvent), ethanol, benzyl alcohol, D4

Image 4: Tear mends using methylcellulose with solvent masks: control (methylcellulose alone with no solvent), ethanol, benzyl alcohol, D4

Methylcellulose, without the aid of solvents, both darkened and lightened the areas along

the join, but did not cause tidelines.

Samples treated with ethanol dried with dark tidelines surrounding a ligher area near the

mend. Mends made with wheat starch paste created sharp tidelines close to the join, while

mends using methylcellulose created tidelines that were more diffuse and further from the

join.

Treatment of the tear edge with benzyl alcohol caused lightening near the join that faded

into undisturbed media without hard tide lines. Results using benzyl alcohol followed by

wheat starch paste were much lighter very close to the join than those made with

methylcellulose, and resulted in the most visible mends.

D4 had little visual effect on tear mends with either adhesive. Mends made using D4

followed by wheat starch paste appeared slightly lighter at the join than mends made

using wheat starch paste alone. The solvent appeared to have a slight ability to prevent

darkening at the join using methylcellulose as an adhesive; however, there was lightening

very close to the join that seemed to be related to treatment of the tear edges with D4.

An unanticipated result was the apparent impermanence of the dyes with quick fading,

possibly as a result of oxidation of one or both of the colorants. Mended samples that

dried under small squares of blotter were protected from fading that occurred on

uncovered portions of the samples. The effect can be seen in Images 3 and 4 where hard

transitions from dark to light surround each mend.

Conclusion The experiment tested the suitability of three solvents for preventing movement of

soluble colorants in paper objects during tear mends, with special attention paid to a

cyclic silicone compound octamethylcyclotetrasiloxane, or D4. D4 was the only solvent

tested that did not cause disruption of water sensitive media when grossly applied. All

solvents tested failed to “hold in place” design media during application of water. Ethanol

and benzyl alcohol accentuated the disturbance while D4 had little effect. The colorant

used to prepare samples, McCormick Red Food Color, faded quickly. A more permanent

dye would be preferable for creating samples that can be referenced over time. Future

testing will use gas chromatography mass spectroscopy to examine the retention time of

D4 in paper substrates.

Bibliography Acros Organics BVBA. 2009. Material safety data sheet 98914. July 20 2009. Fair Lawn, NJ. Banik, Gerhard. and Irene Bruckle. 2011. Paper and water: a guide for conservators. Boston: Elsevier. Cyclododecane: Technical Note on Some Uses in Paper and Objects Conservation Irene Brückle, Jonathan Thornton, Kimberly Nichols and Gerri Strickler Journal of the American Institute for Conservation , Vol. 38, No. 2 (Summer, 1999) , pp. 162-175 Fiske, B. 2013. Personal Communication. Instructor, Winterthur and University of Delaware Art Conservation Program (retired), Winterthur, DE. Ford, J. 2013. Claymont mural cleaning instructions. Winterthur and University of Delaware Art Conservation Program, Winterthur, DE. Hartman, D. 2013. Mural cleaning how-to, Winterthur and University of Delaware Art Conservation Program, Winterthur, DE. Irving, J. 2013. Personal Communication. Instructor, Winterthur and University of Delaware Art Conservation Program, Winterthur, DE. Momentive Specialty Chemicals Inc. 2012. Product stewardship summary sheet: octamethylcyclotetrasiloxane (D4). September 12. Columbus, Ohio. Wolbers, R. 2013. Personal Communication. Art Conservation Department, University of Delaware, Newark, DE.