OBSERVATIONS ON THE DRYING OF PAPER: FIVE DRYING METHODS AND THE DRYING PROCESS
JANE E. SUGARMAN, & TIMOTHY J. VITALE
1 APPENDIX 1
1.1 EXPERIMENT 2: QUANTIFICATION OF THE DRYING PROCESS
Weight measurements were made on a Mettler Balance (Model AE 160) accurate to 10−4 grams. This model has the advantage of visibility from three sides and the top as well as easy access for sample manipulation and photography.
A rigid stainless steel screen supported by a shaped wire stand (also stainless steel) was used on the balance pan to raise the sample into the middle of the chamber. This configuration allowed free movement of air around the sample. Other materials, including Teflon screening attached to Fome-Cor, were tested and found to be sensitive to changes in relative humidity. In related experiments,9 aluminum was also judged unsatisfactory for such work. Stainless steel was selected because it is non-reactive to changes in relative humidity.
Samples were dried in a dry Stabl-Therm oven manufactured by the Blue M Electric Company. The temperature was stable at 105 ± 2°C.
Three samples measuring 8 × 10 cm were cut of each kind of paper (except for sample 3). All samples were subjected to the procedures described below.
The sample, which had been allowed to equilibrate with conditions in the paper laboratory (50 ± 2% RH at 72° ± 2°F) was weighed. The sample was then placed in a bath of deionized water for 2 hours. It was removed from the bath with tweezers, placed on a blotter to remove dripping water, then positioned on the stainless steel screen on the balance. The sample was allowed to air dry. After its weight and visual appearance were recorded it was placed on the balance, and every 5 minutes thereafter until it was approximately the same weight as before wetting.
All of the doors of the balance were left open to facilitate drying. Each time a sample was to be weighed, the doors were closed and the balance allowed to reach equilibrium. The door at the top was then slid open, and a black-and-white 35 mm photograph was taken from above. The weight registering on the balance at the moment of the photo flash (or as close as possible) was recorded. The photographic record has not been used, thus far, in this study.
After air drying, the sample was placed in a Mylar envelope sealed by means of a sonic sealer on three sides. A small loop of stainless steel wire was inserted at the opening of the envelope to hold it open and allow moisture to escape. The envelop was placed in the dry oven at 105°C for 18 hours. After this time, the wire prop was removed from the envelope, and the Mylar envelope was slipped into a zip-locked polyethylene bag. The bag was further sealed by folding it onto itself and held closed with a weight. The sample was allowed to cool for 1 hour. The sample was weighed in the Mylar envelope to determine the “bone dry” weight. From this weight, the percent moisture content at all other stages of the process could be calculated. The formula and specifications for drying were taken from the standard test method ASTM D 644–55 (reapproved 1982). Percent moisture content figures (part of the ASTM method) have been converted to percent solids in order to facilitate comparison with the paper-making literature. (Percent moisture content is not the same as percent water.)
2 APPENDIX 2
2.1 EXPERIMENT 3: PROLONGED HUMIDIFICATION OF SEVEN PAPERS
After experiment 1 (immersion, observed drying process, and oven drying) each sample was removed from the mylar envelope in which it had been oven dried and placed in a paper folder. After several weeks' exposure to ambient laboratory conditions (50 ± 2% RH at 72° ± 2°F), the samples were subjected to prolonged exposure to high relative humidity in a humidity chamber. A stainless steel tray was fitted with a stretched polyester screen suspended over a blotter, which was kept wet throughout the 120-hour period. The tray was kept covered with a ½-in thick Plexiglas sheet. The humidity chamber was monitored with a dial hygrometer, the relative humidity never reached 100% RH but hovered around 95% RH for the entire period.
1. A series of 35mm slides at 0.1x, 3x, and 22x was made of a smooth waterleaf cotton paper dried under a variety of conditions that both increased texture and decreased texture. These were presented in a lecture format to an audience of paper conservators. Many if not most of those in the audience were capable of observing the three different textural domains.
2. Paper conservators know from experience that in drying paper from a flooded state, a “blot off” is required. Without this quick initial blotter change to a smooth blotter, the cockling blotters will distort the object. We believe that the early blotter changes are important to prevent sheet distortion. We also believe that an extended period of drying, after the initial blotter changes, is vital because intermediate blotter changes could fall during the critical period after the onset of physical distortion (at 63% solids, as explained in section 5.3). Determining the ideal length of the slow drying process was not a subject of this study. We believe the several weeks allowed in the early restraint method was probably excessive.
3. The qualities of “surface texture” and “surface grain,” as described in section 3, were not articulated to the trained observers. In retrospect, we recognized that these are the two texture domains that were judged.
4. All of the analytical techniques except for early SEM analysis at Dupont and use of the Photovolt meter at the University of Delaware were carried out at the Conservation Analytical Laboratory, Smithsonian Institution, Washington, D. C., 1986–89. A Photovolt Reflection Meter (model 610), based on a 0°/0° illumination-measurement geometry was made available by Weaver, head of Textile Science Department, University of Delaware, Newark, Delaware. A number of Minolta Chroma Meters were used to measure surface texture using the L∗ parameter of the L∗a∗b∗ color measurement method: model CR-100, 8mm diameter, using d/0° geometry; CR-110, 50 mm diamter, using d/0° geometry; CR-131, 25 mm diameter, using 45°/0° geometry. Total reflectance was measured with a HunterLab Ultra Scan Spectrocolorimeter which uses an integrating sphere. Goniophotometric analysis was carried out with a HunterLab GP 200 Automatic Goniophotometer.Initial SEM examination was performed by Larry Gropp, technician, Dupont Experimental Station, Wilmington, Delaware.Further SEM examination was performed by Walter Brown, head of the Scanning Electron Microscope Laboratory, National Museum of Natural History, Smithsonian Institution, Washington, D. C., using a Cambridge 250 SEM with a LAB6 source. Samples were coated using a sputter coater, instead of a vapor deposition coater, which has been shown inferior due to more charging, for the coating of organic materials such as paper.
5. The work presently under way by Jonathan Amey, using diode array technology, calibrated against a visual standard evaluated by a group of 10 trained observers, shows results that are just below the accuracy of a group of observers. If this work progresses, the technology has the potential to surpass a group of trained visual observers.
6. Porosity measurements included Bendtsen air porosity, bubble count method (diameter of 10% largest pores), and measurement of the average diameter of the 35 largest pores). All tests were performed at the Institute for Paper Chemistry, Appelton, Wisconsin, now the Institute of Paper Science and Technology, University of Georgia, Athens, Georgia.
7. Calculations of mean fiber length and width were performed using a Zeiss microscope at 40x.
8. It should be noted that these results were obtained following a specific wetting and drying history. Each sample had passed through immersion (saturation), air drying, oven drying, equilibrium for several weeks in laboratory conditions, followed by prolonged exposure to high relative humidity. Prior history of mositure gain and loss affects moisture regain capacity as well as other part properties.
9. The insight into the water vapor-aluminum interaction was provided by Timothy Padfield, then environmental specialist, Conservation Analytical Laboratory, Smithsonian Institution, Washington, D. C. Padfield is now a conservation scientist at the National Museum of Denmark, Lyngby, Denmark.
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JANE E. SUGARMAN is a paper conservator in private practice in Greensboro, North Carolina. She is a 1986 graduate of the University of Delaware/Winterthur Art Conservation Program. As a National Museum Act fellowship recipient, she spent two years working with Christa Gaede in her private practice in Arlington, Massachusetts. Address: 512 South Elm St., Greensboro, N. C. 27406.
TIMOTHYJ. VITALE is senior paper conservator at the Conservation Analytical Laboratory, Smithsonian Institution. He has a B. S. in art history from San Jose State University, San Jose, CA, 1973, and a M. S. in art conservation from the University of Delaware/Winterthur Art Conservation Program, 1977. He was a staff member at the Stanford University of Museum, 1970–74 and served an apprenticeship with Sella Petri, a book conservator in San Francisco. He was head of paper conservation, Intermuseum Conservation Association, Oberlin, OH, 1978–82, and chief of presevation branch, National Archives and Records Service, Washington, D. C., 1982–83. Since 1983 he has been at the Conservation Analytical Laboratory. He was president of the AIC Book and Paper group from 1982–84, and served on the AIC certification and accreditation committee 1981–86. Address: Conservation Analytical Laboratory, Smithsonian Institution, Washington, D. C. 20560.