[Note: This paper was originally given at the Fourteenth Annual Art Conservation Training Programs Conference, May 4-6, 1988, at Buffalo State College. It appears in the 1989 volume of "Papers Presented..." at that conference on p. 77-84, under the author's maiden name, Lisa E. Hall.
The paper was one of three given in the session for student papers the following year at the annual conference of the American Institute for Conservation, but was not subsequently published. It is reprinted here, with permission of the Buffalo State training program and the author. Even though most readers are aware that thymol has lost favor as a treatment for mold, they may appreciate the detail in this technical report, because it helps explain why thymol lost favor. -Ed.]
This paper discusses the structure, uses, and hazards of thymol. It then provides an overview of the record in conservation literature of the effects of thymol on the paper support, pigments, and media. It concludes with an experiment designed to test the aging properties of works on paper exposed to thymol. From this particular experiment, thymol appeared to have a deleterious effect on the paper support, on gum arabic, and on iron gall ink.
Although many fungicides have been introduced into the paper, book, and archives conservation field in recent years, thymol continues to be the best known and is still widely used in fumigation and humidification chambers and in starch paste adhesives.
This paper proposes, first, to discuss the structure, uses, and hazards of thymol; secondly, to investigate the effects of thymol on paper, pigments, and media as recounted in conservation literature; and, finally, to describe an experiment designed to test the aging properties of some papers, pigments, and media which were exposed to thymol in a controlled situation.
The Structure, Uses, and Hazards of Thymol . Thymol has a relatively simple organic structure. Its IUPAC name is 5-methyl-2-isopropyl-1-phenol. At room temperature, it is in a colorless, translucent crystalline or flake form. Thymol has a low vapor pressure, a property which keeps it from being effective in an open system and from having any residual fungicidal effect on objects which have been treated. When used according to appropriate procedures in a fumigation chamber, thymol can effectively kill active mold, that is, mold growing on a damp surface. Thymol can also be used as an inhibitor in what otherwise might be an environment conducive to the development and growth of mold, such as in a humidification chamber.
Outside the conservation field, the thymol chamber has had a long and traditional use in libraries and archives. In many institutions, artifacts coming into the collection are routinely placed in the chamber to kill any mold which might be growing on them. As this paper will attempt to demonstrate, such routine use of thymol (especially when the artifact is already dry and any mold is inactive) is, for the most part, useless and may indeed be harmful to the materials comprising the artifact.
At one time, thymol was considered to be nontoxic, and it is still in many situations handled cavalierly. Today, however, thymol is suspected of being a carcinogen and is considered to be moderately toxic. Precautions should be taken when handling thymol: one ought to wear rubber gloves and to handle it only in a ventilated system.
The Literature Search. A search of the conservation literature on this topic revealed that thymol was the only fungicide mentioned in Sir Hillary Jenkinson's Manual of Archive Administration published in 1937. Even then, Jenkinson noted that thymol had, at times, a softening effect on vellum, size, and glue, but concluded that "the use of a thymolized duster can do little harm."
In the past three decades, various controlled experiments, as well as routine examinations of paper objects have resulted in the publication of articles indicating that thymol causes discoloration in paper, parchment, and organic pigments. Ultraviolet light, elevated temperatures, and/or high levels of relative humidity accelerate the observed effects considerably.
What is the mechanism by which works of art on paper discolor after exposure to thymol? One paper dealing specifically with this topic appeared in 1986 in Studies in Conservation. Vincent Daniels, of the Conservation Research Section of the British Museum, and Brian Boyd, of the City of Bristol Museum and Art Gallery, reported on the "Yellowing of Thymol in the Display of Prints." In the Bristol Museum, some prints had been sealed in a framing system with thymol. After three and a half years, it was revealed that the prints had yellowed.
The authors surmised that since phenols oxidize relatively rapidly, that the discoloration present in the paper must be oxidized thymol. There are no reports in the chemical literature mentioning the identity of the photochemical oxidation product of thymol, but thymol can be chemically oxidized. Mass spectroscopic analysis of the discolored thymol provided little evidence for the oxidation of thymol. Rather, molecular mass peaks indicated that thymol itself was present, as well as a polymeric form of thymol, two to five monomer units long. The paper mentioned no further instrumental analysis to confirm the presence or absence of the projected photooxidation products. Daniels and Boyd concluded that only thymol and its polymer were present in the discolored paper.
Although this article did not deal with fumigation per se, it is one of the few published papers addressing specifically the phenomenon of the discoloration of paper in relationship to its exposure to thymol.
To investigate this area further, an experiment was devised to examine the effects of the aging characteristics of paper exposed to thymol. Variables were kept to a minimum. The following six papers were tested:
On the Fabriano paper were painted twenty-two strips of Winsor & Newton watercolors. Among these were included four organic and organo-metallic pigments such as are known to be sensitive to thymol: gamboge, rose madder, Prussian blue, and indigo. In addition to these twenty-two were added two other organic pigments bound in gum arabic: Indian yellow lake and Indian red lake. Also included were pure gum arabic and wheat starch paste. The gum arabic was chosen to test the sensitivity of the binder without pigment. The wheat starch paste was chosen because not only is it commonly used in paper conservation, but it is also regularly stored in the presence of thymol.
All of the papers were divided into sections for testing.
Samples of each of the "new" papers, the newsprint, the filter paper, and the Fabriano paper, were aged in a dry oven at 110°C for five days. The remaining three papers were considered to be naturally aged.
Samples of all the papers, both artificially and naturally aged, were exposed to thymol. The procedure and experimental specifications for fumigation followed the published guidelines. An armature of PVC pipe was constructed. From this, the samples were hung on linen thread. The entire assemblage was placed into a fume hood. Three-tenths of an ounce of thymol, the proportionate amount necessary for the space in the chamber (approximately four cubic feet), was measured into a petri dish and was placed over a 25-watt light bulb. The armature, samples, thymol, and bulb were covered with a sheet of polypropylene and sealed with tape.
The bulb was turned on for three hours per day for four days in a row. The heat provided by the bulb accelerated the sublimation of the thymol. The chamber remained sealed for another three days. At the end of the week of fumigation, the polypropylene sheet, the light bulb, and the petri dish were removed. Published instructions concerning the removal of artifacts from a thymol chamber vary from no instructions at all to allowing the objects to be in a ventilated system for several hours. The purpose of the latter directive is to allow for any residual thymol to sublime so that there is no risk to the operator in handling the fumigated artifacts. The samples were allowed to air for a week in the fume hood with the fan on. This excessive precaution was taken to assure that the paper would be as free of residual thymol as possible. At the end of the week, samples from all six of the papers exposed to thymol were returned to the oven and aged as before. Also aged were samples which had been previously aged but not exposed to thymol.
At the end of the experimental procedure, there were five test samples from each of the new papers:
The nineteenth-century papers had been divided into four test strips:
The paper support. A qualitative comparison of these papers produced, in some cases, rather predictable results. First, exposure to thymol alone, without aging, appeared to have no visible effect on the paper, pigments, or media. Aging alone resulted in discoloration and degradation of the paper and some media. Introduction of thymol into the aging procedure appeared to accelerate the degradative process.
When newsprint samples were examined, the deleterious effect of exposure to thymol appeared in the relatively greater darkening of the sample which had been exposed to the fungicide and subsequently aged compared with that sample which had only been aged twice. The appearance of the other paper samples paralleled the newsprint, although the differences in color were not so dramatic.
The result of the test of the Whatman filter paper, a high alpha-cellulose paper with no sizing or fillers, was among the most revealing. In visible light, the relative darkening of the test samples paralleled the newsprint. However, when the samples were viewed under ultraviolet light, the degradation--apparently of the cellulose--was dramatically revealed. The sample which had been exposed to thymol and then aged was much brighter than any of the other samples. As there are no fillers or sizes present, the brightness of the ultraviolet-produced visible fluorescence is directly proportional to the breakdown of the cellulose chains. While aging alone does appear to affect this paper slightly, exposure to thymol obviously accelerated the degradative process.
Although the answer is not available at this point, the question should be raised: What caused the increased yellowing seen in the samples which were treated and subsequently aged compared with those left untreated and aged? Was it actually the accelerated degradation of the cellulose in the filter paper, and/or the size, in the other samples? Or was it residual thymol or the polymeric or oxidized form of thymol which had chemically and/or physically bonded with the paper? Even though the paper samples were maintained in a ventilated system for a week after exposure to thymol--far beyond the published specifications which assure complete sublimation of the chemical --there is still no assurance that all of the thymol or its oxidation products had left the paper.
The pigments. Conservation literature provides documentation of organic pigments' reacting to thymol in treatments. In this particular experiment, there was no evidence of change in any of the organic pigments. In visible light, change was evident only in the zinc white, which acquired a halo. This is found in naturally-aged watercolors. These test results would demonstrate only that the halo was a result of aging alone; the formation of a halo was not a result of the thymol treatment.
When the organic pigment, rose madder, was viewed under ultraviolet light, it appeared to have been affected by the aging and further affected by the exposure to thymol. Evaluation of this physical evidence indicated that this phenomenon was not a reaction of the lake pigment to the thymol, but rather, it was a response to the treatment of the binder around the pigment and the paper beneath. The fact that the red lake showed no change in the visible light, and that it is among the most transparent of pigments would support this conclusion.
The media. Further evidence lay in examination of the binder itself, gum arabic. The strip bearing the gum arabic alone, without any pigment, had the characteristic appearance of a degraded film. The strip yellowed in proportion to the amount of time that it was aged; that section exposed to thymol and subsequently aged appeared to have yellowed even more. The deterioration of the gum arabic was even more evident in ultraviolet light. The aged film fluoresced; the film exposed to thymol and then aged fluoresced even more brightly. On a molecular level, the reaction which gum arabic undergoes in dry artificial aging appears to involve cross-linking and chain breakdown and reforming. It is believed that the hydroxyl and carboxyl groups react to form ester linkages. The degradation of the gum arabic, it would then appear, was masked by all but the most transparent pigment.
Although rarely used as a binder (such as in paste papers), the reaction of the wheat starch paste is best discussed in this section. The wheat starch paste manifested the same tendencies as the watercolor medium. It should be made clear that, in this particular experiment, the wheat starch paste was taken from the prepared supply in the lab. This was stored in a container in which thymol had been allowed to vaporize in order to control the growth of mold in the paste. In other words, in this particular experiment, there was no example of wheat starch paste, artificially aged, which had not been exposed to thymol. The degradation manifested by the aged samples relative to the others is therefore inconclusive.
The final medium indicating any response to exposure to the thymol treatment is iron gall ink. The ink had been applied to a lightweight laid paper, which, like the Fabriano paper, is of rag content having a gelatin/alum size. Examination of the sample exposed to thymol and aged revealed that the ink on each side of the sample had caused marked discoloration of the paper on the opposite side. There was no such reaction with the sample that had simply been aged twice. Here again, as before, the mechanism by which this degradation takes place is unclear. Exposure to thymol has caused the paper in contact with the ink and/or the ink and/or perhaps residual thymol in contact with the ink to oxidize forming increased discoloration of the paper fibers in close proximity to the ink.
Any conclusions drawn must be qualified by acknowledging that there are many variables not addressed in this experiment: only a limited number of papers, sizes, and binders were tested. There were neither temperature nor relative humidity variables in the artificial aging process. Not addressed was the question of thymol and its effect of the alkalinity of paper, and hence, on its effect on various pigments. Also not addressed were light aging and photochemical degradation. Acknowledging that this simple experiment is in no way a definitive study, it is yet possible to draw some conclusions.
Apparent degradation of the paper support, watercolor binder, and iron gall ink indicates that exposure to thymol damages their structures. The exact mechanism of the degradative process is unclear. Whether the thymol, or some derivation thereof, is retained in the paper is moot; a change does occur which is damaging to the object.
Alternatives to thymol are available, and if at all possible, they should be used. In terms of destroying mold, airing in sunlight and bathing in alcohol are two options available to the conservator that do not require elaborate equipment. Vacuum aspiration appears to remove active mold without leaving the paper damaged. Thymol should never be employed as a fungal retardant in a long-term storage context. Thymol should never be considered to have any residual fungicidal effect. Any conservator faced with treating active mold on artifacts should be apprised of the benefits, hazards, and residual effects of any option chosen. The above investigation would indicate that, if the use of thymol can be avoided, it should be.
For aiding me in the preparation of this paper, I would like to acknowledge the Art Conservation faculty of Buffalo State College as well as Elizabeth Kaiser Schulte of the Conservation Center for Art and Historic Artifacts in Philadelphia. Special thanks go to Cathy Baker and Dan Kushel, faculty members, who gave so freely of their time and expertise.
1. Deborah Nagin and Michael McCann, Thymol and o-Phenyl Phenol: Safe Work Practices (New York: Center for Occupational Hazards, 1982), p.1.
2. Maurice F. Bond and A.D. Baynes-Cope, "Fungicides," Journal of the Society of Archivists 4:1 (1970):51.
3. Nagin and McCann, Thymol, p. 1.
4. Bond and Baynes-Cope, "Fungicides," p. 51.
5. Anon., abstract of "Szkodliwe oddzialywanie srodkow owadobojczych i grzybobojczych na zabytki papierow," ("The Damaging Influence of Insecticides and Fungicides on Paper"), Biblioteka Muzealnictwa i Ochrony Zabytkow 24, Series B (1969): 145-61, by Hanna Jedrzejewska, in Art and Archaeology Technical Abstracts 8:3 (June l971): entry #8-1256, p. 57; Beck, Curt Werner, abstract of "Control of Fungus on Archive Books," Kozarstvi 35:8 (1985): 233-35, by Alois Orlita and F. Martinek, in Art and Archaeology Technical Abstracts 23:2 (1986): entry #23-2155, p. 129.; Hanna Jedrzejewska, abstract of Destrukcyjne procesy w zabytach I. Dzialanie srodkow do ochrony drewna. (Destructive Processes in Antiquities I. The Action of Wood Protecting Agents.) Warsaw: Polskie Towarzystwo Lesne, 1970, by the author of the abstract, in Art and Archaeology Technical Abstracts 8:2 (Dec. 1970): entry #8-759, p. 99; Romuald Kowalik, "Decomposition of Papyrus by Microorganisms,: Restaurator 4:3-4 (1980): 163-70; Irena Sadurska and Romuald Kowalik, "Fungi Preventive for Archival Papers," Bollettino dell' Istituto di Patologia del Libro XXVII:I-II (1968): 37-47.
6. Vincent Daniels and Brian Boyd, "The Yellowing of Thymol in the Display of Prints," Studies in Conservation 31:4 (Nov. 1986): 156-58.
7. Lamp black, Davy's gray, sepia, raw umber, raw sienna, yellow ochre, burnt sienna, gamboge, cadmium yellow, Venetian red, rose madder, Chinese white (or zinc white), viridian, terre verte, Winsor green, Winsor blue, Prussian blue, indigo, cobalt blue, French ultramarine, cadmium red, and vermilion.
8. Anne Clapp, Curatorial Care of Works of Art on Paper, 3rd ed. (New York: Nick Lyons Books, 1987), pp. 78-80.; Nagin and McCann, Thymol, pp. 1-4.
9. Anon., "Szkodliwe oddzialywanie," p. 57.
10. Rutherford J. Gettens and George L. Stout, Painting Materials: A Short Encyclopedia. (New York: Dover Publications, Inc., 1966), pp. 28-29; John S. Mills and Raymond White, The Organic Chemistry of Museum Objects (London: Butterworths, 1987), pp. 66-67; and Jill Sterrett, "Gum Arabic as a Binder in Watercolor Paints and a Preliminary Study into the Factors that May Induce its Insolubility in Water," Senior Research Project, Art Conservation Department, State University College at Buffalo, Cooperstown, NY, August 1986.
11. Sterrett, "Gum Arabic," Conclusion.
12. Mary Wood Lee, "Alternatives to Fumigation: A Review of Techniques for the Removal of Mold Growth from Works of Art on Paper," paper presented at the 16th Annual Meeting of the American Institute for Conservation, New Orleans, 1-5 June 1988; Lee," Removal of Active Mold Growth and Treatment of Structural Damage in Nine Erotic Indian Miniatures," Preprints of Papers Presented at the 11th Annual Meeting of the American Institute for Conservation, Baltimore, 25-29 May 1983, pp. 140-49.
Anon. Abstract of "Szkodliwe oddzialywanie srodkow owadobojczych i grzybobojczych na zabytki papierow" ("The Damaging Influence of Insecticides and Fungicides on Paper"), Biblioteka Muzealnictwa i Ochrony Zabytkow 24, Series B (1969): 145-161, by Hanna Jedrzejewska, in Art and Archaeology Technical Abstracts 8:3 (June 1971): entry # 8-1256, p. 57.
Disinfecting agents can be damaging to sensitive components in paper. Thymol can cause discoloration in certain organic colorants.
Beck, Curt Werner. Abstract of "Control of Fungus on Archive Books." Kozarstvi 35:8 (l985): 233-235, by Alois Orlita and F. Martinek, in Art and Archaeology Technical Abstracts 23:2 (1986): entry # 23-2155, p. 129.
Parchment samples were tested with various fungicides including thymol. Thymol caused discoloration of the treated samples when exposed to ultraviolet light.
Bond, Maurice F., and Baynes-Cope, A.D. "Fungicides." Journal of the Society of Archivists 4:1 (1970): 51-52.
This technical note discusses the necessity of climate control and the availability and advisability of various fungicides.
Clapp, Anne. Curatorial Care of Works of Art on Paper, 3rd ed. New York: Nick Lyons Books, 1987, esp. pp. 78-80.
An outline of a thymol fumigation procedure is given. Included is a description and diagram for a thymol chamber.
Collis, Ivor P. "The Use of Thymol for Document Fumigation." Journal of the Society of Archivists 4:1 (1970): 53-54.
The use of thymol as a fungicide for bulk treatments is discussed. The chamber, equipment, and precautions used are described.
Daniels, Vincent, and Boyd, Brian. "The Yellowing of Thymol in the Display of Prints." Studies in Conservation 31:4 (Nov. 1986): 156-58.
Experimental work was done to determine the cause for rapid yellowing of prints displayed in frames containing thymol. Mass spectroscopic analysis revealed the presence of a polymer thymol; there was no evidence for the expected photoxidation products.
DerSarkissian, Michael, and Goodberry, Mayda. "Experiments with Non-Toxic Anti-Fungal Agents." Studies in Conservation 25:1 (1980): 28-36.
In this study, thymol was compared with three non-toxic anti-fungal alternatives.
Gettens, Rutherford J., and Stout, George L. Painting Materials: A Short Encyclopedia. New York: Dover Publications, Inc., 1966, esp. pp. 28-29.
The structure of gum binders is covered.
Gibson, J.A., and Reay, D. "Drying Rare Old Books Soaked by Flood Water." Museums Journal 80:3 (Dec. 1980): 147-48.
The process of drying water-soaked books is discussed. Thymol was used to discourage fungal growth.
Jedrzejewska, Hanna. Abstract of Destrukcyjne procesy w zabytach I. Dzialanie srodkow do ochrony drewna. (Destructive Processes in Antiquities I. The Action of Wood Protecting Agents.) Warsaw: Polskie Towarzystwo Lesne, 1970, by the author of the abstract, in Art and Archaeology Technical Abstracts 8:2 (Dec. 1970): entry # 8-759, p. 99.
The author found that after some weeks, thymol could cause visible destructive effects in sensitive materials. Humidity, ultraviolet light, and elevated temperatures accelerated the observed effects considerably.
Kowalik, Romuald. "Decomposition of Papyrus by Microorganisms." Restaurator 4:3-4 (1980): 163-70.
In the course of a larger study, thymol and other fungicides were tested. It was demonstrated that thymol has no residual effect. Samples exposed to thymol exhibited a decrease in mechanical strength.
Lee, Mary Wood. "Alternatives to Fumigation: A Review of Techniques for the Removal of Mold Growth from Works of Art on Paper." A paper presented at the 16th annual meeting of The American Institute for Conservation, New Orleans, 1-5 June 1988.
The paper discussed various small tools and techniques used in the process of mold removal, including the construction and use of the vacuum aspirator.
________, "Removal of Active Mold Growth and Treatment of Structural Damage in Nine Erotic Indian Miniatures." Preprints of Papers Presented at the 11th Annual Meeting of the American Institute for Conservation, Baltimore, 25-29 May 1983, pp. 140-49.
The paper discusses the successful use of a vacuum aspirator to remove active mold.
Lukens, R. Chemistry of Fungicidal Action. New York: Springer-Verlag, 1971.
Reactions involved in fumigation are covered.
Mills, John S., and White, Raymond. The Organic Chemistry of Museum Objects. London: Butterworths, 1987, esp. pp. 66-67.
The structure of gum binders is covered.
Nagin, Deborah, and McCann, Michael. Thymol and o-Phenyl Phenol: Safe Work Practices. New York: Center for Occupational Hazards, 1982.
Description, use, and precautions necessary in handling thymol are discussed. Description and plans for a thymol chamber are included.
Parsons, D.E. "Fumigation of the Site." Repairers' News Sheet, Society of Archivists, England, No. 18 (Jan. 1971):1.
This documents a mass fumigation procedure using thymol. A list of the equipment used is included.
Sadurska, Irena, and Kowalik, Romuald. "Fungi Preventive for Archival Papers." Bollettino dell'Istituto di Patologia del Libro XXVII:I-II (1968): 37-47.
The authors investigated the chemical, mechanical, and biological resistance of paper treated with fungicides, including thymol mixed with mercuric chloride. The effectiveness of the fungicides on mold and on the pH of the paper is documented.
Sax., N. Irving. Dangerous Properties of Industrial Materials. 4th ed. New York: Van Nostrand Reinhold Co., 1975.
This gives general information and hazards analysis on thymol.
Schulte, Elizabeth Kaiser, Conservator, Conservation Center for Art and Historic Artifacts, Philadelphia, PA, telephone conversations, 26 and 28 April 1988.
The advantages, disadvantages, and methods of using various fungicides were discussed, as well as the possible effect of thymol on the pH of cellulosic materials.
Stecher, Paul G., ed., et al. Merck Index. 8th ed. Rahway, NJ: Merck & Co., Inc., 1968.
This gives general information and hazards analysis on thymol.
Sterrett, Jill. "Gum Arabic as a Binder in Watercolor Paints and a Preliminary Study into the Factors that May Induce its Insolubility in Water." Senior Research Project, Art Conservation Department, State University College at Buffalo, Cooperstown, NY, August, 1986.
The structure and the possible aging mechanism of gum arabic binder are discussed.
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