IMPROVEMENTS IN THE MANU- FACTURE OF IMITATION SLATE OR COMPOSITION ESPECIALLY APPLICABLE FOR WRITING ON
Year: 1879

page_text: PROVISIONAL SPECIFICATION left by Peter Jensen at the Office of the
Commissioners of Patents on the 21st March 1879. (A communication from
abroad by the "Vereinigte Gummiwaaren Fabriken, Harburg-Wien," late
"Menier J. N. Reithoffer," of Harburg, Germany).
PETER JENSEN, of 33, Chancery Lane, London. "IMPROVEMENTS IN THE MANU-
FACTURE OF IMITATION SLATE OR COMPOSITION ESPECIALLY APPLICABLE FOR
WRITING ON." [A communication from abroad by the "Vereinigte Gummiwaaren
Fabriken, Harburg-Wien," late "Menier-J. N. Reithoffer," of Harburg, Germany.]
The imitation slate writing tablets hitherto in use are made of paste board
covered with a thin coating of a mixture of ground pumice stone, bone black, and
linseed oil varnish. Such tablets do not answer the purpose well, their surface
being too soft, they retain permanent impressions when written on with a hard
slate-pencil; besides they are not fit for use as soon as the thin coating is worn off.
This Invention consists in the manufacture of a homogeneous material which is
not brittle, and which is superior to slate as to lightness; and especially in this
respect, that the lines and figures produced thereon by a slate pencil are whiter and
more visible than on common slate. Furthermore the dull surface of this imitation
slate or composition does not blind the eye by reflected light, so that wooden
boards veneered with the new composition are very suitable for tablets hung on a
wall, as blackboards in schools, lecture rooms, and other places.
In the manufacture of this composition pumice stone and bone black are ground
in suitable machines and mixed in machines, such for instance as are used in
india rubber factories, with purified india rubber in the following proportions:-
10 parts by weight of india rubber.
16 parts by weight of powdered pumice stone.
21 parts by weight of bone black.
5 parts by weight of sulphur.
Powdered pumice stone may be replaced by powdered flint, powdered glass, or any
other material suitable for producing a rubbing surface. Instead of bone black,
any other material giving a black or dark color may be used.
The composition is then rolled out into thin sheets, cut to size, and put into piles
in the following way-First, a layer or sheet of tinplate; then a layer or sheet of
paper; then a layer or sheet of the above composition; then a layer or sheet of
Damian 931

page_text: paper; then a layer or sheet of tin plate, a layer or sheet of paper, a layer of the
above composition, a layer of paper, a layer of tin plate, and so on.
The piles thus formed are strongly compressed by means of screws, weights, or
cramps, and brought into a boiler, where they are heated by steam for about
24 hours at a temperature of about 260 to 290° F. Then the piles are taken out
of the boiler; each single layer of the composition covered by the paper on both
sides is pressed between two tables which are heated by steam, and again exposed
for about 2 hours to a temperature of about 260 to 290° F.
When the sheets of the composition have been cooled they are polished with
pumice stone and ready for use.
Sheets destined to serve as veneerings to wall tablets receive before the heating
à cover of linen or similar adhering material on one side, in order to facilitate the
adhesion of the sheet when glued to the wooden boards.