ALVAR AND BUTVAR: THE USE OF POLYVINYL ACETAL RESINS FOR THE TREATMENT OF THE WOODEN ARTIFACTS FROM GORDION, TURKEY
KRYSIA E. SPIRYDOWICZ, ELIZABETH SIMPSON, ROBERT A. BLANCHETTE, ARNO P. SCHNIEWIND, MAURAY K. TOUTLOFF, & ALISON MURRAY
4 ALVAR AND BUTVAR: PROPERTIES AND USES
Both Alvar and Butvar belong to the family of polyvinyl acetal resins that are formed by the reaction between aldehydes and alcohols. In industry, the reaction is carefully controlled to produce a variety of polymers containing predetermined proportions of hydroxyl, acetate, and acetal groups (Skeist 1977). Polyvinyl acetate, polyvinyl alcohol, and polyvinyl butyral are examples of polymers produced by this process.
Alvar was obtained by replacing the acetyl radicals in polyvinyl acetate by acetaldehyde. This resin was widely available in Great Britain and North America during the 1950s and 1960s. A brief overview of the uses of Alvar and other polyvinyl acetal resins in conservation is provided by Horie (1987). Alvar served conservators as a component of a widely used gap-filling material for ceramics known as AJK Dough. In addition, it became a favorite of paleontologists for the consolidation of fragile fossil material. However, its manufacture ceased by the mid-to-late 1960s (Howie 1984).
Product literature dating from 1949 describes the Alvar class of resins as “exceptionally hard and tough.” Similarly, strength and toughness are cited as advantages of the polyvinyl butyrals in the literature of a current manufacturer (Solutia Inc. 1999). Common solvents for the polyvinyl butyrals include alcohols, glycol ethers, and certain mixtures of polar and nonpolar solvents. Alvar was soluble in a similar range of solvents. Payton selected the polyvinyl butyral resin Butvar B-98 on the basis of being able to achieve a low viscosity in solution when a solvent mixture of ethanol and toluene was employed. Glass transition temperature may also be an important factor in choosing a resin for use in hot climates. The glass transition temperature of Butvar B-98 falls between 72°C and 78°C, which is well above that of either Paraloid B-72 (40°C) or PVA resins (18-29°C). The high glass transition temperature of Butvar B-98 proved particularly advantageous when the decision was made in later years to employ Butvar B-98 as the primary adhesive in reconstructing the wooden artifacts from Gordion.