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There are a number of factors which will determine the likelihood of coated paper sticking together:
In order to be able to answer these questions it is necessary to understand something about the nature and composition of coated papers.
"These papers include those made with specially treated pulp stock or having highly finished surfaces resulting from the application of coatings or impregnation with oils and resins. Examples of objects with these properties include drawings on papers with prepared grounds, chromolithographic prints, transparent papers, and varnished papers, such as maps. These types of papers pose special conservation problems owing to particular properties imparted by the various processing treatments." 1
"Coatings are usually applied to paper and paperboard for the following purposes:
(1) To achieve uniformity of surface, either to the eye or to the inks, lacquers and/or varnishes applied from a printing press.
(2) To enhance opacity (either waxed, dry or varnished), smoothness, and the gloss of paper or paperboard.
(3) To achieve economy for base paper furnishes and to save where the brightness or functional properties of certain coatings may permit the use of mechanical pulps or unbleached pulps or waste paper." 2
These coated papers are commonly found in glossy art books, textbooks containing illustrations, magazines etc.
Basic components for papers designed for the graphic arts:
Most common pigments are:
Pigments normally use polyphosphates; carbonates are dispersed with modified sodium hexametaphosphate (Calgon T); auxiliary dispersants may be sulphonated naphthalene formaldehydes, sodium terpolymers, phosphated potassium copolymers etc. Casein, soy protein, and oxidized starch can also be used.
Baker et al also discuss the composition of coated papers and list a number of fillers that have been used:
ground bone ash, white lead, calcium carbonate, gypsum, powdered cuttlebone, wax, zinc oxide, titanium dioxide, acrylic gesso, clay, talc, satin white (aluminum hydroxide, calcium hydroxide, and calcium sulphate), barium sulphate, calcium sulphate, calcium sulphite, or calcium carbonate.
They list binders used as:
starch, mucilage, gums (i.e. gum arabic), gelatin and animal glues (hide glue), casein, modern latexes and synthetic resins (i.e. acrylic emulsions), peanut and soybean proteins.4
Vacuum freeze-drying is the process by which water is removed from materials by means of a vacuum. By lowering the pressure inside the freeze-drying chamber, the solid ice particles in the material are directly converted into water vapour in the process known as sublimation. By-passing the liquid water phase minimizes further damage from the movement of water within the material. Vacuum freeze-drying is most successfully employed on objects such as books, or bird and mammalian specimens which are difficult to air dry thoroughly, and for items on coated paper.
Objects to be freeze-dried are first pre-frozen, and removed from plastic bags to facilitate the freeze-drying process. However, books, which are usually packed close together in boxes or plastic milk crates, should be separated from each other with wax or freezer paper to prevent the covers from sticking together. They should placed in the crate spine down. Plastic-coated binders, should be pricked first to prevent them from blowing up like a balloon when the vacuum is pulled.
Vacuum freeze-drying is the preferred and most effective of the freeze-drying methods for drying large quantities of water-damaged paper, but it is also the most expensive. A typical procedure may involve:
| The vacuum freeze-drying process at CCI takes place in a Virtis 36DX66 unit at -20° to -25°C for an average of 7 days (depending on the quantity of material and degree of wetness) without using heat. Items are normally weighed first and at intervals throughout the procedure to monitor moisture loss. |
Vacuum thermal drying is less expensive than vacuum freeze-drying. It is frequently confused with vacuum freeze-drying. During vacuum thermal drying:
"Books on coated paper present special problems. Coatings are usually applied to paper in order to obtain uniformity of surface, to enhance opacity, smoothness, and gloss. The basic components of coatings contain pigments, such as china clay or a solubilized protein. Waters notes that in the presence of water, starch-based coatings and some casein mixtures may revert from dry adhesive to gel and then back to the solid on drying. When these adhesive mixtures are in a fluid state, any pressure will cause the coatings to weld together and create a permanent bond during the drying cycle."6
"The problem of blocking or adhesion of leaves to each other is primarily confined to books and periodicals printed on stock that uses a coating pigment with a binder of casein and starch, two highly water-soluble substances. If wetted material of coated stock is permitted to dry it will turn a book to a clay-like brick. Restoration is impossible. At the moment the only known salvage method considered practical, especially where large quantities of water-damaged material are involved, is freezing while wet, then freeze-drying."7
"Materials printed on coated paper. If allowed to dry closed, such materials will congeal irreversibly into solid blocks of paper."8
Flieder states that:
"Les feuilles n'ont jamais collé ensemble, même dans le cas de papier couché, ce qui est tout à fait exceptionnel, car séchées à l'air, elles auraient formés des blocs indissociables."9
Both Flieder and Walsh recommend keeping coated paper wet:
"Coated papers will stick together unless frozen or dried immediately. Keep them wet in cold water until they can be air dried or packed for freezing."10
"Nous signalerons enfin que la lyophilisation est également une méthode de sauvetage des document qui séchés à l'air libre, se sont agglutinés sous forme de "briques" compactes. Afin de récoupérer les documents ainsi perdus, la méthode consiste à les immerger pendant plusieurs heures dans de l'eau, puis à les lyophiliser. On pourra alors, dans la majorité des cas, séparer le feuilles les un des autres."11
In my opinion, this would cause the filler to become more soluble and cause further damage to the paper. Walsh goes on to say that when drying documents printed on coated paper:
"Note that freeze drying has a far better success rate than air drying. If the papers are wet, separate each coated paper from the other by applying a sheet of polyester and lifting the plastic away with the paper. At this point, the paper can be dried on the polyester, which can be hung on lines. Alternatively, dry the partially wet paper by interleaving between every sheet with waxed paper, or laying individual sheets on polyester web covered blotters."12
There is however, consensus that ultimately, books and documents on coated paper should be frozen, then freeze-dried.
"Many coated papers can be difficult to dry without sticking together once they are wet. Because it is nearly impossible to determine which papers will block, all coated papers should be treated the same way for the purpose of vacuum freeze drying: before any drying takes place, and ideally within 6 hours of becoming wet, materials should be frozen at -10° F or lower. They may then be vacuum freeze dried with a high potential for success."
And,
"The advantages of vacuum freeze-drying for books and paper are that there is less distortion...and a good recovery rate for coated papers."13
Some conservators caution against the use of vacuum drying (vacuum thermal drying) and only recommend vacuum freeze-drying:
"In most cases the only chance of saving books and materials printed on coated stock is to first freeze them when wet, followed by vacuum freeze-drying or vacuum drying."14
"Coated papers should never be vacuum dried because they will stick together."15
"Vacuum thermal drying - coated paper tends to block irretrievably."16
There is, however, consensus that coated paper should not be air-dried:
"If an efficiently air-conditioned room can be established with the capacity to maintain a constant relative humidity of 25 to 35 percent and temperatures between 50° and 65° F, books with only wet edges can be dried successfully in approximately two weeks without interleaving. No attempt should be made to dry books printed on coated stock by this method. In nearly every case, the only chance of saving such materials is to freeze them while wet and return to the dry condition by vacuum freeze drying."17
"Air-drying...is seldom successful for drying bound, coated paper."18
In a paper published by George M. Cunha in 1977, evaluating the mass drying of books Cunha cites the experimental work of Fischer and Duncan and their observations on the drying of coated and uncoated papers:
"(a) Drying techniques that work well on uncoated paper do not necessarily work well with coated stock;
(b) Freeze/thaw/vacuum drying technique is particularly effective with catalogs and journals containing coated paper;
(c) Water penetration in coated book pages may not be uniform, particularly if books were tightly packed on shelves;
(f) The one type of coated paper used in this study absorbs 80% more water than the one type of uncoated paper used;
(j) Absorption of water by coated paper can be more rapid than that for uncoated paper;
(n) Quickness in freezing coated-paper books after recovery from a disaster helps to reduce blocking i.e. sticking together of pages - upon subsequent drying. Blocking can take place before freezing;
(o) Blocking is sometimes caused by uneven or excessive heating in mass drying operations. The thawed ice dissolves starch and casein, constituents of some coated-paper stock. Both being adhesives, they then stick the pages together during the final stages of drying. Evidence exists that this happens when the temperature in a book exceeds 10° C. However, when temperatures are kept below 4.4°C, there is no sticking as a result of the freeze-thaw drying operation;
(p) Rewetting blocked book pages and immediately freezing, followed by drying, sometimes makes it possible for the stuck pages to be separated;
(q) Microwave and dielectric energy has not been successfully applied to drying of books printed on coated paper--i.e., starch, casein, clays, etc.
(s) Uncontrolled heating by microwave and dielectric energy can cause coated-paper pages to balloon (or burst). Water inside each page is converted to vapor more rapidly than it diffuses to the surface of each page."19
In Fischer's experiments set up to simulate the conditions under which books become wet during disasters he found that:
"The loosely packed coated- and uncoated-paper books absorbed more water than the tightly packed books--approximately five times as much for the coated- and twice as much for the uncoated-paper books. In both series of experiments more than 93% of the water absorbed in the loosely packed, uncoated-paper books evaporated during a 120-hr drainage period. These experimental results could explain why several of the actual frozen, flood-damaged, uncoated-paper books, after thawing, contained only small amounts of water. They were perhaps the last uncoated-paper books to be packed and frozen after the flood water receded."20
He later goes on to say:
"The loosely packed, coated paper books under equivalent 120-hr drainage conditions lost approximately 67% of the absorbed water. It was difficult to remove the water from this type of coated-paper book regardless of the drying technique. If the book was not frozen, the individual pages had to be interleaved shortly after removal from the flood waters; otherwise, severe sticking would occur and make diffusion of the remaining water to the outside edge of the book difficult."21
Fischer concluded that:
"The freeze-thaw, vacuum-drying process appears to have great promise for drying not only uncoated- but coated-paper pages. The results of drying the paper in the coated-paper books are significant; only those processes rated above 40 dried the coated paper without producing the sticking problem. The other processes with a rating of 31.2 or below during drying caused severe sticking."22
And:
"Freeze-thaw vacuum, interleave-air, and solvent extraction processes offer the greatest potential in drying these types of coated- and uncoated-paper books after they have been wetted and frozen."23
While it appears that there exists very few adequate scientific studies on the effects of the various freeze-drying techniques on water-damaged coated paper in the conservation literature, there are numerous papers and articles published in conservation journals and books. The opinions discussed here on the methods of drying coated papers damaged by water following an emergency or disaster are those of various leading book and paper conservators working in North American libraries and archives. While there are some slight variations in methodology, it is generally agreed by these experts (and other book and paper conservators working in the field) that coated paper will stick together on drying, on account of the coating or "loading" (china clay, kaolin, titanium dioxide) becoming soluble in water then fusing together with adjacent pages as it dries.
It is also generally agreed by book and paper conservators that this type of paper should be frozen as soon as possible after it has come into contact with water, then vacuum freeze-dried, to prevent against sticking. Failure to take immediate action will result in either partial or total adhesion of the paper. After that, no amount of freeze-drying will reverse the process. It should be pointed out that in the case of Fischer's experiments, vacuum (thermal) drying was used ("freeze/thaw/vacuum dry with controlled heat assistance") as opposed to vacuum freeze-drying, which does not use heat but relies on sublimation (see explanation of the two freeze-drying techniques) and although he claims that vacuum drying was successfully used on only one type of coated paper tested, other experts have stated that this process is not recommended and that adhesion will occur.
According to the literature the recommended ideal time within which water-damaged coated paper must be frozen is 6 - 8 hours if it is to be salvaged without further damage occurring.
1. Baker, Mary, Dianne van der Reyden, & Nancy Ravenel, "FTIR Analysis of Coated Papers", in: The Books and Paper Group Annual (Washington DC: American Institute for Conservation of Historic & Artistic Works), vol.8, 1989, p.1.
2. Loomer, Joseph T, "Coated Papers", in: Handbook of Pulp and Paper Technology, Kenneth W. Britt (ed) (New York: Van Nostrand Reinhold Company), 2nd edn., 1970, p.517.
4. Baker, et al., op. cit., pp. 1 - 2.
5. Tremain, David, and Deborah Stewart. Emergency & Disaster Preparedness for Museums. Accompanying manual for CCI workshop. Canadian Conservation Institute, 1996 (unpublished).
6. McCleary, John M. Vacuum Freeze-Drying, A Method to Salvage Water-Damaged Archival and Library Materials: A RAMP Study with Guidelines (Paris: UNESCO), 1987, p.4.
8. Barton, John P. & Johanna G. Wellheiser. An Ounce of Prevention: A Handbook on Disaster Contingency Planning for Archives, Libraries and Record Centres (Toronto: Toronto Area Archivists Education Foundation), 1985, p. 60. [This publication is currently being revised]
9. Flieder, Françoise, Claire Chahine, and Martine Leroy. "Traitement de masse des documents graphiques en cas d'inondation", in: Recontres internationales pour la protection du patrimoine culturel. 2ème colloque. Les risques naturel (Avignon: Centre de congrès du Palais des Papes), 1987, p. 143.
10. Walsh, Betty. "Salvage Operations for Water Damaged Archival Collections: A Second Glance", in: WAAC Newsletter (Santa Monica, CA: Western Association for Art Conservation), vol.19, no.2, May 1997, p.15.
11. Flieder, et al., op.cit., p.145.
13. Buchanan, Sally. Emergency Management. Drying Wet Books and Records. (North Andover: Northeast Document Conservation Center), 1994. NEDCC Technical Leaflet, p. 2.
14. Barton & Wellheiser. p. 68.
16. Buchanan, Sally. "Disasters - what bad news, what good news", in: The Book and Paper Group Annual (Washington DC: American Institute for the Conservation of Historic & Artistic Works), 1986, vol. 5, p.175.
17. Waters, Peter. Procedures for Salvage of Water-Damaged Library Material (Washington DC: Library of Congress), p.21.
18. Buchanan. Emergency Management. p.1.
19. Cunha, George M. "An Evaluation of Recent Developments for the Mass Drying of Books", in: Preservation of Paper and Textiles of Artistic Value, John C. Williams (ed.) (Washington DC: American Chemical Society), 1977. Advances in Chemistry Series, 164, Ch.7, pp. 95 - 104 . [The author cites: Fischer, D.J., Duncan, T, "Conservation Research: Flood-Damaged Library Materials", Bulletin of the American Institute for Conservation (1975), vol.15 (2), pp. 28 - 48; Fischer, D.J., "Simulation of Flood for Preparing Reproducible Water-Damaged Books and Evaluation of Traditional and New Drying Techniques", Preservation of Paper and Textiles of Artistic Value, John C. Williams (ed.) (Washington DC: American Chemical Society), 1977. Advances in Chemistry Series, 164, Ch.8, pp. 105 - 123; Fischer, D.J., "Conservation Research: Use of Dielectric and Microwave Energy to Thaw and Dry Frozen Library Materials", Preservation of Paper and Textiles of Artistic Value, John C. Williams (ed.) (Washington DC: American Chemical Society), 1977. Advances in Chemistry Series, 164, p.124.]
20. Fischer, David J. "Simulation of Flood for Preparing Reproducible Water-Damaged Books and Evaluation of Traditional and New Drying Techniques", Preservation of Paper and Textiles of Artistic Value, John C. Williams (ed.) (Washington DC: American Chemical Society), 1977. Advances in Chemistry Series, 164, Ch.8, p. 109.
1. See: Lagrenade, Marcel. Vocabulaire Technique. Fabrication et Transformation du Papier. Anglais-Français. De l'entrée à la sortie de l'usine (Domtar Inc) 1981, 6th edn., p.76; Van Derveer, Paul D. & Leonard E. Haas (eds). International Glossary of Technical Terms for the Pulp and Paper Industry. English, Svenska, Deutsch, Français, Español.(San Francisco: Miller-Freeman Publications Inc), 1989, 5th edn., p.36.
The opinions or interpretations of the information expressed in the document are mine, or those of the authors who I am quoting; I did not consult with them during the writing of this document. I did not request, or receive, permission to quote them. The document was written purely in response to a specific enquiry I received from a client. This document is not an official CCI document or publication and has not received the approval of CCI management to publish it. Neither does it necessarily reflect the opinions of other conservators and conservation scientists at CCI.