Raw materials
1. Scraps of hide of animal
2. Lime
3. Zinc oxide
Process
1. Hides are washed to remove dirt and are soaked to soften them. This material is called stock.
2. The stock is passed through a series of water baths in which more and more lime is added to make the hides swell and break them down. The swollen hides are rinsed in a large washing machine to remove the lime. The last traces of lime are eliminated by treating the stock with a weak acid (acetic acid). Finally, the stock is cooked either by boiling it in open tanks or cooking it under pressure in autoclaves.
3. Cooking at the correct temperature and for the right length of time breaks down the collagen and converts it into glue.
4. To remove impurities and make the glue clear, chemicals like alum or acid followed by egg albumin may be added. These chemicals cause the impurities to precipitate.
5. Zinc oxide is added to produce white "school glue."
This glue is different from poster glue. Different raw materials and much more involved process of making it. With certainty it would be more expensive than poster glue. Accidentally, like glasses and papers, there would be many different kinds of glue: glue for paper, for wood, for plastic, for metal. Interesting. Needless to say the raw materials and processes to make each one of them would be different!
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