This section is from the "A Complete Dictionary of Dry Goods" book, by George S. Cole. Also available from Amazon: A complete dictionary of dry goods and history of silk, cotton, linen, wool and other fibrous substances,: Including a full explanation of the modern processes ... together with various useful tables.
Tortoise-Shell (Tortis-Shell). The scales on the outer shell of certain sea-turtles. These horny scales or plates are naturally of a beautiful mottled or clouded color, and are extensively used in the manufacture of combs, brushes, card-cases, hair pins, and trinkets. The shells are sometimes of enormous size, measuring a yard across. The value lies in the color more than in size, though large and small are sold together by weight. A clear, amber-colored shell is considered most valuable, and this, in its rough state, is worth $10 per pound; red-brown is next in point of value, at $6 per pound, then dark brown at $4 per pound. The shells are principally worked up in Naples and Messina, Italy, where they undergo a great deal of polishing and cleaning before being put into use. Olive oil and a composition called tripolo are used for polishing, and these produce a smoothness and luster which bring out all the beauty of the shell. A great deal of time and labor is devoted to this, for the Italian does not know the good service of machinery, but works slowly and well with small tools. With the aid of minute knives and files, he takes a piece of shell and patiently goes to work. He seems to do it lazily, rather indifferently, but his eye is exact from long practice and what is done is well done, and will bear critical inspection. There are no rough edges, no unpolished portions. The same care is taken in every detail, and laziness and slowness must needs be forgiven in view of the result. The scales become very plastic when heated; and when allowed to cool retain with sharpness any form they may be molded to in the heated state. Pieces can also be welded together under the pressure of hot irons. The best tortoise shell is obtained from the Indian Ocean. It is in this country largely imitated in horn, and in artificial compounds of much less cost, none of which are so clear or flexible as the genuine shell.