Milk casein is an excellent glue for paper to itself or to glass and for wood.
With a paintbrush, coat the paper to glue and fix it to the other substrate (paper, cardboard or glass).
The best results are obtained with skimmed milk (0% fat) and with powdered milk: for instance, dissolve 1 oz. (35 g, about 6 tablespoons) of skimmed powdered milk into one cup (200 ml) of hot water. Add while mixing a few tablespoons of white vinegar or lemon juice until the milk curdles and precipitates casein. Filter through two layers of cheesecloth and rinse with cold water. Press the cheesecloth to remove as much liquid as possible. Transfer to a container, cut the mass in small pieces and add up to 2 fl. oz. of a 1% ammonia solution (if commercial solution has a concentration of 12% ammonia, an approximate 1% solution can be obtained by mixing a tablespoon of 12% ammonia in ¼ cup of water or 50 ml). Mix with a wooden spoon or a blender until the mixture becomes fluid and viscous.
Sodium bicarbonate can be used instead of ammonia. Add a tablespoonful of bicarbonate to 4 tablespoons of hot water and add to the curd. The curd will foam due to the release of carbon dioxide gas (CO2).
Best results are obtained by following these precautions:
- Milk must be hot to very hot when adding acid.
- As much water as possible should be removed from the curd.
- Divide the curd in as many small pieces as possible.
- If the curd is too viscous and difficult to mix, add a small amount of hot water. Be aware that if the glue contains too much water, it will be less sticky and will take a longer time to dry.
- If the casein is left for a few hours, it will become more fluid and transparent. However, it will not keep longer than one or two days.
Cheese is made from milk curdled with an enzyme (rennet) or by lactic fermentation.
An example of the use of casein glue which was significant but is now declining is the manufacturing of wine bottles corks. For a long time it was the only stable naturally-derived glue that did not alter the taste of wine on contact. It is used to make composite agglomerated corks or caps that have rings cork glued to another medium. They constitute the cork stopper which is in direct contact with the wine.