LETTERS TO THE EDITOR
1.1 To the Editor:
Barbara Appelbaum's article “Criteria for Treatment: Reversibility” in JAIC 26:2, Fall, 1987, reflects the author's refreshing frankness and practicality and gives us all much to discuss and think about. However, I would like to refute some of her statements regarding reversibility of cleaning on pages 66 and 67. In my opinion, both Hedley in his work which she cites, “On Humanism, Aesthetics and the Cleaning of Paintings,” and Appelbaum have missed an essential point. There is no such thing as “total cleaning” of paintings. There is usually an interactive zone between original varnishes (or replacement varnishes) and paint films that is not even or continuous; it varies over various pigments, etc.
Intuitive picture cleaners have been aware of this zone effect for generations and understandably shied away from “squeaky clean” paint surfaces as goals. Now Richard Wolbers has begun to show and demonstrate this phenomenon in many cases through fluorescent staining of microscopical sections. Oil paint surfaces (or even the surfaces of wood furniture) can show an intermingling effect with surface coatings. If one stains for resin one can find resin in the coating and in the uppermost crust of the oil film or wood. If one stains for oil one can find oil in the oil film or wood and in migrating “fingers” into the surface coating (in furniture this is migration of essential wood oils). Given this state of things, a constant drive toward “total cleaning” can lead to overcleaning in some cases.
The supposition that cleaning is “visually reversible” should not be accepted in all cases, especially if anyone has seen a demonstration of photographic “filtering” to replace the complex visual effect of a discolored varnish on a painting put side by side with the actual painting before cleaning. There is a very distinct difference in veracity, integrity, and translucency. I believe Richard Wolbers' work has suggested even to those who may not acknowledge it visually, that there is indeed such a thing as & do I dare say it? – patina on paintings. It is the interactive zone.Joyce HillStonerDirectorArt Conservation Program, University of Delaware, 303 Old College, Newark, Del. 19716
1.1 To the Editor:
Though I am not directly involved with research in paper conservation, Antoinette Dwan's article (Spring '87) regarding paper complexity did prompt me to read more literature on the subject.
For such a comprehensive article a discussion of the origin of the physical properties of cellulose would have been appropriate. In fact, the reader was somewhat misled concerning this relationship when Coulomb's law was discussed.
Figure 4 on page 5 gives a hint to the fundamental reason why cellulose (a polysaccharide) is such a fibrous, insoluble material. The chemical -OH groups as illustrated for cellobiose (a disaccharide) create what is known as hydrogen bonding with -OH groups from other molecules (intermolecular hydrogen bonding). Though weak as compared to other forces, multiplied by many times over the whole system of polysaccharides this attractive force leads to the overall strength of cellulose; and why these materials are fibrous in nature, and not easily dissolved or melted. Strictly speaking, Coulomb's law applies only to charged particles and not materials composed of covalent bonds. (Even though in the case of polarized covalent bonds some atoms may take on a slightly charged character.)
Further, in the section headed “Moisture-Temperature Relationship” it was stated that below 240°C cellulose behaves as an elastic material and above 240°C becomes viscoelastic. This temperature is identified in the article as the glass transition (Tg) of cellulose. Though this may be true it is important to mention other thermal properties of cellulose. According to the work cited by Pearce, et. al, (Thermal Characterization of Polymeric Materials, E. Turi, Ed. Academic Press, : 815–816) pyrolysis of cellulose occurs as low as 200°C. Pyrolysis is an irreversible chemical change that turns cellulose into dehydrocellulose which eventually leads to a darkened char. When this type of chemical degradation precedes or occurs during the Tg of any material, the glass transition loses all practical significance. Hence it is inappropriate to presume viscoelastic (fluid-like) properties of cellulose above 240°C when it chemical integrity is being compromised.
In retrospect, even though a few phenomenological inconsistencies exist, the article stands as a vast resource of information of the paper conservator. Furthermore, it is successful in illustrating the complexities involved when interpreting the behavior of material with such a wide range of both chemical and physical variables.ChristopherMcGlincheyThe Metropolitan Museum of Art, Fifth Avenue at 82nd Street, New York, N.Y. 10028
Response to McGlinchey's letter to the editor:
The above comments are based on a view of paper as purely a cellulosic material. One of the significant points of the article was the composite nature of paper and the fact that cellulose chemistry alone does not supply all the information on reactions of paper as a material. The heart of the discussion was the interaction of all furnish materials (cellulose, lignin, hemicellulose, extractives, sizing agents, fillers, coatings, dyes, etc.) with formation processes (pulping, forming, drying, calendering, etc.) to create the composite matrix of paper. Although very important, concentrating on the origin of the physical properties of cellulose would have been a tangent to this main topic.
As states on page four, bonding in paper is an active area of current research that extends beyond the explanation of intermolecular hydrogen bonding. Readers are referred to the work of D.H. Page and his colleagues of the British Paper and Board Industry Research Association.
Coulomb and Hooke's laws are cited as fundamental material on paper texts as background information on physical properties of paper and paper testing. It was presented to briefly describe the molecular relationship to crystalline/elastic and amorphous/viscoelastic regions in paper. Coulomb and Hooke's laws do apply to the crystalline regions of paper, and are the basis for physical measurements of paper. Ultrasonic testing and x-ray diffraction are two other testing methods currently used that are based on the crystalline of paper.
The glass transition temperature, Tg, in paper is not calculated through thermal tests but by physical tests where force determines the Tg. Due to the use of paper as a material (printing, hanging, manufacturing, etc.) this method of determination is preferred, and the topic of extensive research. Readers are referred to: N.L. Salmen and E.L. Back, “Moisture-Dependent Thermal Softening of Paper, Evaluated by its Elastic Modulus,” TAPPI, Vol. 63. The temperature cited for dry cellulose serves as a reference for the amount of variability due to changing moisture content in paper. Moisture acts to alter the proportion of elastic to viscoelastic properties and therefore affects measurement of those properties. The reported measurements of the glass transition temperature for paper vary, depending on testing conditions (such as moisture content), and methodology. The main point was not a specific temperature, but to explain why, and to what extent, paper can recover from deformation and return to an original state.