Bone glues are usually darker than hide glues, but some bone glues go through an artificial clarifying process which gives them the appearance of high-grade glues but really detracts from their strength. Very frequently oxide of zinc is added to glue, the effect being to make it set quickly, as well as to give it a light color. Some glues contain so much oxide of zinc that they are milk-white. Zinc oxide is not harmful except when added in very large quantities.
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.
BREAKING QUALITY
This is a simple test that affords an important indication of the quality of glue. Take a small piece between the thumb and forefinger of each hand and bend it. A very thin piece of good glue will bend without breaking. When it does break, if the edges are splintery, great tensile strength is indicated. A clean fracture, on the other hand, indicates a brittle, low-grade glue, which has been subjected to heat so long as to destroy the tissue; or else it has been made from bone stock. High-grade glues never show glassy fractures, but bone glues do. In making this test, the air conditions of the room should be taken into account. If the glue has been kept in a dry room it will naturally break much more readily than if it has been in a moist atmosphere. This is especially important to bear in mind if comparative tests are being made.
FOAM
A simple test for foam is to beat a solution of glue with an ordinary eggbeater. Glue which shows foam, or in which foam does not quickly subside, probably contains impurities. Foam is especially frequent in alum-dried glues and in cheap bone glues.
Some authorities believe that foam is caused by overheating, due to scalding by contact with steam jacket, or by steam coming into direct contact with the glue, or by heating for too long a time, or it may be due to the fact that all the grease has not been eradicated.