NOTE ON A FILL MATERIAL FOR WATER SENSITIVE OBJECTS
Conservators have become familiar with glass microballoons primarily as used in air abrasive or peening techniques. They have been used in conjunction with other fill materials, primarily to reduce thermal expansion of setting resins.1
Described as hollow, unicellular soda lime borosilicate glass microspheres,2 this product was developed for industrial use in syntactic (homogeneous, closed-cell) foams, injection molding and autobody repair. The dry microspheres are lightweight, resembling a fine white powder, and the 3M products range in density from 0.15 to 0.40 grams/cm3. They pour like a liquid, and are non-toxic.3
As the glass will not react with most resins,4 it may have a wide range of application in conjunction with materials commonly used by conservators. The spherical shape of the particles allows for a low percentage of resin in the formulation of fill material, and ensures relatively low shrinkage. The bubbles are easily mixed into resins. (They may rise to the top of a mixture during storage, but are readily redistributed by stirring.) The products formulated for general use guarantee that 95% of the particles will be 80 mesh (177 microns) or smaller. Microballoons are available in a range of densities and respective compressive strengths.