EFFECTS OF DILUTE CALCIUM WASHING TREATMENTS ON PAPER
JOHN BOGAARD, & PAUL M. WHITMORE
APPENDIX: EXPERIMENTAL DETAILS
The Whatman no. 42 filter paper used in most of these tests was from two reams of large-size sheets (58 x 68 cm) obtained in 1978, which were stored in different environments. Consequently there is some variation in the starting DP of the samples. Monitoring of the changes in DP and carbonyl groups over this storage period indicates that the paper has deteriorated primarily by an acid hydrolysis mechanism; there does not appear to be oxidation along the chain. The one exception to the use of these two reams is in the alkaline degradation of an unoxidized sheet, which was performed on a sheet from a freshly obtained ream (46 x 57 cm) of Whatman no. 42 that has a lower DP. Personal communication with Whatman Inc. (Heilweil 2000) suggests that the lower DP with this ream is a result of natural variation in the starting materials and not any change in the manufacturing process. Generally the untreated and treated samples were cut from the same sheet, with the exception of the exposure to near-ultraviolet light. In this case the hydroxide-treated samples came from one sheet, while the other three sets came from another.
The treatment baths were made to 0.625 millimolar in deionized water that was boiled to minimize dissolved carbon dioxide. The calcium hydroxide and calcium chloride solutions were prepared by dissolution of the appropriate salt. For the calcium bicarbonate solution, a solution of calcium hydroxide was first prepared, and then carbon dioxide was bubbled through it from a reservoir of dry ice while the pH was tracked. From published dissociation constants (Butler 1964), bicarbonate formation occurs when the pH reaches a plateau at 6.4. The bubbling was immediately stopped at this plateau, which took only a couple of minutes to reach. The pH of the calcium hydroxide treatment solution was 10.4, and the pH of the chloride treatment solution was 6.2. The calcium hydroxide treatments of unoxidized samples before accelerated thermal aging and near-ultraviolet exposure were carried out under a nitrogen atmosphere in order to avoid interference from carbon dioxide with the treatment solution. Treated sheets were then dried in the open air. The other treatments were handled completely in the open air. Treated sheets were held at room temperature for at least five days before analyses and aging.
Calcium content of the sheets was determined by inductively coupled plasma atomic absorption spectroscopy (ICP-AA) and is expressed in ppm by weight, which is equivalent to mg/kg. ICP-AA is a precise technique; when replicate measurements were made, they varied by less than 10%. However, the natural variation between sample sheets and in their pickup of calcium during treatment was greater than the precision of the technique. Untreated filter paper varied in calcium content from 13 to 71 ppm. The treated sheets that were used varied from 190 to 230 ppm. These results were used primarily to assemble sample sets of equal calcium loading prior to accelerated aging.
Alkaline degradation was carried out in 1 N sodium hydroxide having a pH of 12.9. photo-oxidized sheets used in alkaline degradation were exposed for 1,536 hours to daylight fluorescent lamps. Other photo-oxidation pretreatments were carried out by exposure for 164 or 168 hours to ultraviolet-A lamps. Lamps used for accelerated light aging were either the Q-Panel UVA-351 for nearultraviolet exposure or the Sylvania F48T12/D/HO for daylight fluorescent. During exposure, the sheets were held 4 in. from the light source. At this distance, radiometer measurements show that the daylight fluorescent lamps produced about 1,800 footcandles, while the ultraviolet-A lamps produced approximately 6 milliwatts/cm2 intensity in the 300–400 nm wavelength range.
The procedure for reduction of photo-oxidized samples was to immerse the sheets in 1% sodium borohydride (pH 9.5) for 15 minutes, a period expected to be sufficient to reduce carbonyl groups on the chain but to have little effect on the more slowly reacting reducing ends of the cellulose polymer. The immersion was followed by thorough rinsing and overnight drying prior to the calcium hydroxide treatment.
Viscometric degree of polymerization of the samples was determined by the standard method (ASTM 1962) in a solution of 0.5 M cupriethylenediamine, following overnight treatment in unbuffered 0.01 M sodium borohydride to stabilize the alkali-sensitive linkages. Scissions can be derived from this DP by first converting it to the number-average degree of polymerization and then calculating the concentration of chains. Expressing the number of chains in the same concentration units (mmol/100g) as the carbonyl and carboxyl analyses, and incorporating the ratio of DPv/DPn for this cellulose-solvent system, the concentration of chains is then equal to 1235/DPv. The difference in the concentrations of chains between unaged and aged samples gives the concentration of scissions that have occurred. These calculations assume random chain breaking is occurring and therefore are not valid for endwise peeling reactions, and they also become less accurate as the leveling-off DP is approached. For a more detailed explanation of these calculations, see Whitmore and Bogaard (1994). The precision of the viscosity measurements is such that changes in scissions of more than 0.1 unit are considered significant.
Carbonyl contents were assayed using the hydrazine method (Albertsson and Samuelson 1962); reported values are the average of two duplicate measurements. The duplicate measurements increase the precision of this technique to approximately plus or minus 10% of the value. Carboxyl contents were determined using the standard methylene blue test (ASTM 1963); even without duplicates this technique is precise to about plus or minus 10% of the value. The methylene blue carboxyl test is, however, unable to detect carboxyls that form on soluble fragments of the degraded cellulose polymer. Measurements of the cold extraction pH initially followed the standard method (TAPPI 1988), using degassed, deionized water; later, the modification proposed by Scallan (1990), using a 0.1 N solution of sodium chloride in degassed, deionized water, was adopted.
This work was performed at the Research Center on the Materials of the Artist and Conservator at Carnegie Mellon University. The financial support of the Andrew W. Mellon Foundation is gratefully acknowledged.
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SOURCES OF MATERIALSCupriethylenediamine
GFS Chemicals Inc. P.O. Box 245 Powell, Ohio 43065 (800) 858-9682 www.gfschemicals.comLab supplies; Whatman no. 42 filter paper (large sheets)
Fisher Scientific Co. 585 Alpha Dr. Pittsburgh, Pa. 15238 (800) 766-7000 www.fishersci.comOther chemicals
Aldrich P.O. Box 2060 Milwaukee, Wis. 53201 (800) 771-6737 www.sigma-aldrich.comSpectrophotometer (Color-Eye Model 7000)
Macbeth Division Kollmorgen Instruments Corp. 405 Little Britain Rd. New Windsor, N.Y. 12553Temperature and humidity chamber
Blue M Electric 2218 W. 138th St. Blue Island, Ill. 60406 (708) 385-9000Daylight fluorescent lamps (F48T12/D/HO)
Grainger Industrial Supply Local branches nationwide 1-800-CALL-WWG www.grainger.com
UV-A fluorescent lamps (UVA-351)
Q-Panel Corp. 26200 First St Cleveland, Ohio 44145
JOHN BOGAARD has a B.S. in chemistry from Carnegie Mellon University. He has been with the Research Center on the Materials of the Artist and Conservator since 1978, where he is an associate staff scientist. His primary area of research has been paper chemistry. Address: Carnegie Mellon Research Institute, 700 Technology Drive, Room 3210, Pittsburgh, Pa. 15219.
PAUL M. WHITMORE has a Ph.D. in physical chemistry from the University of California at Berkeley. Following an appointment at the Environmental Quality Laboratory at Caltech studying the effects of photochemical smog on works of art, he joined the staff at the Harvard University Art Museums. Since 1988 he has been director of the Research Center on the Materials of the Artist and Conservator at Carnegie Mellon University, where his research has been directed toward the study of the permanence of modern art and library materials. Address as for Bogaard.