above:Tortoiseshell glass
vase made c. 1880s
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Tortoiseshell glass 
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Tortoiseshell Glass: A short explanation:
Tortoiseshell art glass is hand-blown glass in which two layers of blown glass have pieces of brown glass trapped between them. The brown glass is usually in two colors, light and dark brown, and shaped in blotches to resemble the marking on a tortoise, as in the vase shown left.
The original patent for this glass was registered in London by Francis Pohl and S. A. Wittman in 1880 (October 25th). They were from Europe and tortoiseshell glass was made in Germany in the 1880s. Their patent described the process of first blowing bulbs of brown glass and breaking them into fragments onto a hot marver (usually a heated steel or iron plate); then blowing two bulbs of clear glass, one of which was rolled over the brown fragments so that they stuck to the outside of the molten gather of glass. The other bulb of molten glass then had its top cut away so that the bulb with brown pieces could be inserted and the two bulbs joined and blown as one to the required shape. The final product was then coated to produce a yellow stain before firing and annealing (allowing to cool slowly).
This very complicated process produced a beautiful type of art glass, and as usual with 19th century inventions, others invented simpler processes or copied the original. The yellow staining, for example, was not needed if one of the glass bulbs was a pale honey gold colour. There are reports of tortoishell glass being reproduced in Italy recently, but the products were described as being very thick and clumsy.
Tortoiseshell art glass was made in Europe and also in the USA at the Boston and Sandwich Glass Co. in Massachusetts, from 1880 to about 1900. It is generally agreed that distinguishing which factory made a particular item is impossible unless there is a label or makers mark, and these are not usually present on Tortoiseshell glass.
Tortoiseshell was also a name sometimes used for pressed glass with an amber background and darker brown streaks. Sowerby of Gateshead in England introduced Tortoiseshell pressed glass in 1882, but it is rarely found. Davidson's of Gateshead, England introduced a version of their Cloud Glass in the 1930s which they called Tortoiseshell.
Tortoiseshell Glass became popular following the wave of great enthusiasm in the 19th century for products made out the shell of a small sea turtle called the Hawksbill. Genuine tortoiseshell was replaced by tortoiseshell glass, bakelite and plastic in making dressing table and household items, and this change probably saved the little turtle from extinction.
Tortoiseshell glass from Isle of Wight Glass
Michael Harris if Mdina and Isle of Wight Glass invented a glass named "Tortoiseshell" which was very popular in the 1970s and 80s. This is hand blown art glass which acquires the Tortoiseshell colouring from rolling clear molten glass on enamels including silver nitrate. There are good examples in Mark Hill's book.
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