Shellac
Shellac has been
used for centuries in lacquer form as a
protective and decorative finish for wood and
metals (not to be confused with Chinese
lacquerwork that is based on resin from Rhus
verniciflua trees). The lac insect is a plant
parasite producing an exudate as a protective
barrier against predators. Traditionally scraped
off twigs with the eggs, larvae and pupae and
insects, the shellac is cleaned and filtered to
produce a hard, brittle thermoplastic material.
This is used as the basis for sealing wax.
In the nineteenth century many mixtures and
compositions were based on shellac, the most
successful being the American ones of Peck,
Halvorson, and Critchlow. Wood powder was the
filler mixed into the molten shellac to produce a
composition or mixture capable of being moulded
by heat and pressure. They made Union Cases to
hold early daguerreotypes and ambrotypes. Over
1000 different Union Case designs are known,
predominantly black in colour, although red brown
and other dark shade examples exist. The moulded-in
designs vary from the machine-made strictly
geometric, through stylised floral to the
pictorial, frequently based on established
popular paintings. Sizes vary from 1/16th plate
to the full plate 190 x 237 mm. Shellac slowly
thermosets above 100 degrees C, and most shellac
Union cases are largely no longer thermoplastic
as the reaction continues even at room
temperature.
As photographic technology improved, so the
requirement for folding cases diminished and
conventional decorative 'picture' frames were
produced. Many Union cases have the manufacturers'
name and patent details printed on paper and
glued to the inside, behind the image, but very
few have moulded in trade marks.
As the technology shifted to mineral filled
shellac, trade names became more frequent, e.g.
Smith's Patent American Composition of solid back
frames. Dressing table sets with brush and mirror
backs in black shellac compositions (Diatite, etc.)
are not uncommon, occasionally with patent
numbers or dates moulded in. In England, Manton
patented his composition and used it on black
brooches of various designs.
Mineral filled shellac was the material used to
make 78rpm records.
|
