Practical Tests of Glue

This is an excerpt from HANDWORK, I’m still a fair way from finished but here is sample of what to expect in the magazine.

Much has been said of late about the testing of glue, its value and adhesive qualities, different grades and makes. Several rules and regulations have been devised to determine the exact value of each glue which are all very good. Nevertheless, most of these tests can only be made with the aid of special appliances made for this purpose. With this fact in mind the writer will give a few simple rules to determine qualities of glues.

First of all, the odour of glue is always considered a good indication of quality. The best grades of glue have a not too pleasant odour but the cheaper grades are practically obnoxious. Usually the cheap bone glues are easily detected through their rank or nasty smell and can be thus judged for their quality in their dry state. Particularly is this true of the powdered or ground variety.   Another test for ground glue is by taking a handful and closing your hand over it.   If it gets sticky within a minute or two it is a cheap kind, or made of bones, etc.   A good hide glue usually retains its brittle nature for quite a while without getting sticky.  A fact worth mentioning is that cheap ground glues always will be found to lump or pack together in warm weather.
Flake glue will act in most instances the same way as ground glue. Another test for flake glue is to take a piece and try to break it.  If it breaks easily and with a brittle crack it is a sign of cheap glue.  On the other hand, if it proves elastic and can be bent back and forth and is hard to break, it is of a much better quality.

When a piece of good glue is held against the light it should show a perfect
clearness and even texture.  A cheap glue will show streaks and appear cloudy. Glue, however, should not always be judged by colour, as some of the best glues are of the dark coloured variety.

That white glue is stronger in adhesive qualities than the natural colour is an error, as the white glue is always found to be coloured with mineral pigments. White vitriol or oxalic acid and oxide of zinc are most commonly used for this purpose. This is done solely for the purpose of providing a white coloured glue which in some trades is absolutely necessary.  The mixing of any colouring matter has never been found to increase the adhesive quality of the glue.

Glue may be tested for quality as follows:  A thin sheet must bend until both ends touch without breaking. If after breaking a sheet of glue the fracture appears in splinters then such glue has not been cooked properly. If the sheet breaks readily then the glue is weak in binding quality and low in value.
The surface of good glue should be velvety, but sometimes dust falls upon the damp glue, imparting a matte appearance.  Never purchase glue that has been exposed to moisture, nor glue that emits an odour similar to that of dead animals.
Irregular bubbles noted in melting glue, when of large size, prove that the material from which such glue was produced had been in a state of decay.  If, in cooking, glue of this sort emits a noxious stench it is unfit for use.  Even when soaking this glue before melting the disagreeable odour may be noticed.  Further proceedings with glue of that peculiarly objectionable odour would only prove to be a waste of time and money, because such glue when used for veneering or glue jointing will blister or come loose in a short time after or joints come apart when such glue is decaying.
R. O. Neubecker