[CHAPTER VII.]
METHODS OF BLEACHING GLUE.
Many experiments have been made to bleach glue, i. e., to obtain masses as colorless as possible, or at least slightly colored, the resulting product being more valuable than the dark-colored one.
a. Bleaching in the Air.
The principal requisite for obtaining a beautiful bleached glue is that the unbleached product is clear, i. e., transparent, even if of a dark color, this being the best criterion of well-made glue.
Glue may be bleached whilst being prepared, or the finished cakes may be subjected to the bleaching process.
For the purpose of obtaining pale-colored glue from skin or cartilage it is advisable to expose the materials in thin layers to the direct action of the sun. Moist oxygen when acted upon by the sun is converted into ozone, which exerts an extraordinary bleaching effect upon organic substances.
b. Bleaching with Chlorine.
The powerful bleaching effect of a solution of chlorine in water upon organic matter is well known; the water is decomposed, and bleaching is effected by the oxygen which is liberated. Hence skin and cartilage may also be bleached by placing them in a vessel filled with weak solution of chlorine in water and leaving them in contact with it until the fluid shows no longer an odor of chlorine. When bleaching is finished the materials are suspended in a certain quantity of hydrochloric acid, which has finally to be removed by repeated treatment with water.
c. Bleaching with Animal Charcoal.