Although gelatine glue is strong and fast-acting, it does have some significant limitations. It must be applied fresh, hot, and quickly; this makes it impossible to use on large or complex joints, and difficult to use in general. To overcome this, I decided to make a glue based on another common protein: casein. Casein is an alkali-soluble form of milk protein, and can be precipitated from (preferably skim) milk by the addition of vinegar. Historically this would then be dissolved with quicklime (calcium oxide) to make glue, but this would then harden in its container within a day. Fortunately, a shelf-stable liquid glue can be made if ammonia is used instead.
For convenience and accuracy of measurement, I decided to use micellar casein powder (available as a protein supplement) as my source of casein, and clear household ammonia (3-5% ammonium hydroxide) to dissolve it. The dissolution is very slow, with the casein tending to clump and form a skin; furthermore, it cannot be accelerated by heat without denaturing the casein and producing obnoxious fumes. However, if the mixture is stirred occasionally and kept covered otherwise, the reaction will be complete within a day or two. After extensive experimentation, the maximum concentration of casein I was able to achieve was 10% by weight, or a 1:9 ratio of casein to ammonia. This appears to be limited by the solubility of the ammonium caseinate rather than by a shortage of alkali, since all samples contained excess ammonia. Fortunately, despite its low concentration, this solution still makes an effective glue.
The glue is slippery and brushes onto wood easily, and remains fluid for much longer than gelatine, since it solidifies due to evaporation rather than cooling. As it dries it gives off weak fumes of ammonia, gradually reverting back to insoluble casein. The glued joints can be handled within an hour, but reach full strength in a day, and when dry they are fairly water-resistant. Mechanically, the glue is approximately half as strong as gelatine, with the cantilever test used in the gelatine glue article failing at just over 15 pounds of weight. This is likely due to the low casein content causing the glue to shrink considerably as it dries. This is somewhat beneficial however, as any excess glue tends to be pulled back into the joint during this shrinkage, resulting in a fairly minimal glue line.
Overall this is an interesting adhesive, and I will continue to experiment with its properties in order to maximize its strength and water-resistance. Like gelatine, it can be used as both glue and sizing (to seal wood before varnishing), but it also has another practical use which I will explore in the next article: it can be made into paint.