Staining to Make Urea-Formaldehyde Resin Visible on Glued Wood Surfaces

Sixteen staining techniques to provide contrast between droplets of normally transparent cured urea-formaldehyde resin and wood were tested. Stains included spray reagents for thin-layer chromatography, pH indicators, histological stains, and acid treatments. Five of the sixteen staining treatments produced an adequate contrast between resin and wood. These were the Ginzel and Stegmann technique, the Seifert technique, the lignin solvent, and the 10-percent solutions of nitric acid or hydrochloric acid. Because resin droplets were possibly degraded or lost with the Ginzel and Stegmann technique, the Seiffert method and the 10-percent solution of nitric or hydrochloric acid gave the best results. The nitric acid solution was easiest to use and the wood surface appeared to be little degraded. The acid treatments can be applied to freshly sprayed UF resin or to UF resin after curing with the same results. The five treatments that produced adequate contrast between resin and wood appeared to stain even those resin droplets not visible to the unaided eye. Many very small droplets could be seen at higher magnifications under the stereomicroscope.

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