ALKALINE OR ACID QUALITY
The best glues are neutral as to acid and alkali. Glues with an excess of acid should be avoided, especially when used with oak or chestnut or other woods with strong acid qualities, as the acids in the glue may unite with those in the wood in such way as to have a destructive effect upon the glue. In such cases the glue will granulate after a time and the work will pull apart.
When a wood is being used that is strong in acid it is advisable to use a glue containing enough oxide of zinc to neutralize the acid in the wood. In making sizing for paper a glue containing either acid or alkali in excess should be avoided. It is also held by some authorities that acid in glue tends to bring about decay.
To test for alkali or acid, dissolve a small quantity of glue in water and dip a piece of litmus paper into the solution. Acid will turn the paper violet or red. Alkali will turn it blue. Litmus paper may be procured at any drug store.