Glueline Identification

Nondestructive techniques are presented for field identification of wood adhesives. These are based on the appearance of the glueline and the use of a special indicator for determining the relative glueline pH. To the naked eye and through a 10 or 20 power hand lens, the highly alkaline protein glues are distinctive in that they show a marked stain in the wood adjacent to the glueline. In general, cold press glues exhibit a much wider band of stain than do hot pressed adhesives. The highly alkaline phenol-formaldehyde adhesives also show a stain, but considerably narrower than that of protein glues. Typical urea, urea-melamine, melamine, PVA, polyester, and epoxy gluelines are nearly the same color as the wood. The epoxy and polyesters are often transparent, the epoxy yellow-brown in color and the polyester colorless. Resorcinols are dark colored, but exhibit little or no staining. Under ultraviolet light, protein adhesives will exhibit an orange or rose fluorescence. The epoxy and polyester systems will fluoresce, most generally with a green or white fluorescence. The fluorescence of the epoxy glueline is less brilliant, but still visible. Another test which can be field applied is a staining technique to determine relative glueline pH. The special mixed indicator is composed of 0.25 grams of Methyl Orange, 0.25 grams of Bromcresol Green and 0.01 grams of Indigo Carmine. These are dissolved in 50 grams of denatured alcohol and then 25 grams of water are added. This indicator shows a strong red color at pH below 4. It is yellow between 4 and 5, green from 5 through 8, and tends toward a bluish-black above 9. Application of a drop or two of this indicator will show a color change on the glueline and in the adjacent wood. A useful table of field procedures summarizes the conclusions which can be drawn based on glueline color, wood stain, pH and appearance under ultraviolet, light.

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