Process of and Apparatus for the Production of Grained Glass Plates or Screens for Photographic Purposes.
Year: 1898

page_text: Date of Application, 22nd Apr., 1898
Complete Specification Left, 30th Dec., 1898- Accepted, 25th Feb., 1899
Process of and Apparatus for the Production of Grained Glass Plates
or Screens for Photographic Purposes.
I, ALEXANDER MELVILLE CLARK, of the Firm of A. M. & Wm. Clark, of
53, Chancery Lane, in the County of Middlesex, Fellow of the Chartered Institute
of Patent Agents, do hereby declare the nature of this invention (a communication
to me by the Firm of Haas, of 4, Taubenstrasse, Frankfort-on-Main, Germany,
Glass-etcher) to be as follows:-
Heretofore grained glass plates or screens have been made either by engraving
in regular lines, or by etching by means of hydrofluoric acid, the latter process
being effected by first dusting the plates with asphalte, then melting the asphalte,
and afterwards etching the uncovered places by means of the vapour of
hydrofluoric acid.
Grained plates or screens produced according to the last mentioned process
present an irregular graining which for most photographic purposes is preferable
to that of the geometrically grained screens. But it was not hitherto possible by
means of this process to produce plates of a greater size than, say, 7× 7c.m.,
and it was also not possible to produce the etchings with such sharpness as to
render the screens fully equal to the geometrically ruled screens. Now according
to the present process screens can be made of any desired size and with all desired
sharpness.
This end is attained. by the use of a special dusting process whereby a uniform
distribution of the asphalte dust, in settling, is effected over the whole dusted
surface, and by special expedients in the etching operation.
The dusting operation is effected in a closed space in such a manner that only
the finest particles of dust are allowed to settle upon the glass plate under
treatment. In a closed space a dusting powder composed of a mixture of asphalte
and "dragon's blood" is thrown up into the condition of a whirling cloud by
stirring or agitating. The cloud of dust thus produced rises up along a vertical
channel or passage connected at top with the dusting chamber so that only the
very lightest particles pass into this second chamber, on whose bottom is provided a
rotating support for the glass plate or screen to be grained and along with which
there also rotates a fine-meshed sieve. Upon the plate so treated the dust is
melted in the usual manner. The drawbacks of the etching operation hitherto
employed are considerably counteracted by uniformly heating the plate whilst the
latter is exposed to the vapours of the hydrofluoric acid, the vapours being
prevented by this means from condensing upon the plate. The improved arrange-
ment has also for effect, to cause the vapours to pass over and not under the
plate. The result is to produce a comparatively weak and accurately distributed
action of the hydrofluoric acid upon the uncovered portions of the glass plate,
violent impact of the vapours upon the plate being entirely obviated, whilst any
accumulation of liquid hydrofluoric acid upon the plate, resulting in the eating
away of the glass under the dusted places, is prevented by the admixture in the

page_text: asphalte of dragon's blood" or of a similar resin, as well as by the heating of
the plates.
The process is carried out in practice in the following manner by means of the
apparatus shewn in the accompanying drawings.
Referring to Figure 1, A is the dusting chamber which is connected by means
of the vertical channel B with the dust-stirring device C. In the chamber A
there is arranged a rotary disc 1) serving as a support for the glass plate to be
treated, the disc D is mounted on an axle d and is adapted to be rotated by means
of clockwork a. The sieve E rotates as one with disc D. The mixture of finely
powdered asphalte and "dragon's blood" is introduced into the stirring device C
wherein the revolving arms c, by means of the leather pallets e and brushes f,
stir the dust and throw it upwards. Only the very finest particles pass out of the
channel B into the chamber A and passing through the sieve E settle upon the
plate carried on the disc D. The plate thus treated is heated in any desired
manner so as to produce adhesion of the dust by its melting on the plate.
Further, the back of the plate receives a coating of a suitable varnish whereby it
is protected from the action of the hydrofluoric acid. The plate thus prepared is
placed in the apparatus shewn in section in Figure 2. F is the vessel from which
hydrofluoric acid gas is generated by the action of the flame f¹. The gas rises
through the chamber G, at the upper end of which is placed the glass plate Z
to be etched, the plate being disposed in any suitable manner (for example it may
be simply laid upon its back) so that the hydrofluoric acid gas must pass along
between it and the wall g of the chamber on its way to the uptake H. Below the
glass plate Z is arranged a second heating flame h, the heated gases from which
are caused to impinge upon the back or underside of the plate Z so as to heat the
latter sufficiently to prevent condensation of the hydrofluoric acid vapours upon it.
Dated this 22nd day of April 1898.
A. M. & WM. CLARK,
Chartered Patent Agents, 53, Chancery Lane, London.
Process of and Apparatus for the Production of Grained Glass Plates
or Screens for Photographic Purposes.
I, ALEXANDER MELVILLE CLARK, of the Firm of A. M. & Wm. Clark, of
53, Chancery Lane, in the County of Middlesex, Fellow of the Chartered Institute
of Patent Agents, do hereby declare the nature of this invention (a communication
to me by the Firm of Haas, of 4, Taubenstrasse, Frankfort-on-Main, Germany,
Glass-etcher) and in what manner the same is to be performed to be particularly
described and ascertained in and by the following statement : —–
Heretofore grained glass photographic screens have been made either by
engraving in regular lines, or by etching the plates, previously dusted over with
asphalte powder, with the vapour of hydrofluoric acid.
Screens thus etched present an irregular grain which for most photographic
purposes is preferable to that of the ruled screens, but it has only been possible
hitherto to produce etched plates of a very limited size say 7×7 c.m. and it
was not possible to obtain such sharpness of etching as to render the screens fully
equal to the ruled or geometrically grained screens.
Now according to the present process screens may be made of any size desired
and with all requisite sharpness. This result is obtained by a special method of
dusting whereby a uniform distribution of the asphalte dust is effected over the
whole surface of the plate and by special expedients in the etching operation,

page_text: The dusting operation is effected in such a manner that only the finest particles
of dust are allowed to settle upon the glass plate. Within a closed chamber a
powder composed of a mixture of asphalte and dragon's blood." is agitated and
thrown up into the condition of a whirling cloud of dust which ascends through a
vertical passage connected at top with the upper part of a dusting-on chamber so
that only the very lightest particles pass into this second chamber wherein the
glass plate to be grained is supported beneath a fine-meshed sieve upon a rotating
table. The plate having been so dusted-on the dust is caused to adhere by being
melted on in the usual manner.
The drawbacks of the etching operation as hitherto practised are considerably
counteracted by uniformly heating the plate whilst the latter is exposed to
the vapours of hydrofluoric acid, which will thus be prevented from condensing
upon the plate, the arrangement for heating the plate being such that the
vapours only pass over and not under the plate. The result is to produce a com-
paratively weak and accurately distributed action of the hydrofluoric acid upon
the uncovered portions of the glass plate, violent impact of the vapours upon the
plate being entirely obviated whilst condensation of liquid acid upon the plate
and the consequent eating away of the glass under the dust particles is prevented
by the heating of the plates, and by the admixture with the asphalte of dragon's
blood" or of a similar resin.
**
The process is carried out in practice in the following manner by means of the
apparatus illustrated in the drawings accompanying my Provisional Specification.
Referring to Figure 1, A is the dusting-on chamber which communicates by
means of the vertical channel B with the dust stirring-up chamber C. Within
the chamber A the glass plate is supported on a rotary table 1) mounted on a
vertical axle d adapted to be rotated by means of clockwork a, a sieve E above
the plate being mounted on and rotating as one with table D. The mixture of
finely powdered asphalte and "dragon's blood" is introduced into the stirring-up
chamber C wherein leather pallets e and brushes f carried by revolving arms c
mounted on a horizontal shaft driven exteriorly by a belt, stir the dust up into a
whirling cloud and project it upwards so that only the very finest particles pass
up through the channel B into the chamber A where they pass through the sieve E
and settle upon the plate carried on the table D. The plate after being thus
treated is heated in any suitable manner so as to produce adhesion of the dust by
melting it on the plate, and the back of the plate is coated with a suitable varnish
to protect it from the action of the hydrofluoric acid. The plate thus prepared is
placed in the apparatus shewn in section in Figure 2 wherein F is the vessel from
which gaseous hydrofluoric acid is generated by the action of the flame f¹ and
rises through an inclined passage G at the upper part of which the glass plate Z
to be etched is placed, the plate being arranged in any suitable manner (for
example it may be simply laid upon its back) on the lower side of the channel G
so that the hydrofluoric acid gas must pass between its upper face and the wall g
of the passage on its way to the uptake H. Below the support on which the
glass plate Z is laid there is a second chamber heated by a flame h from which the
gases are caused to impinge upon the back or underside of the plate support so as
to heat the plate Z sufficiently to prevent condensation of the hydrofluoric acid
vapours upon it.
Having now particularly described and ascertained the nature of the said
invention and in what manner the same is to be performed I declare that what
I claim is:-
1. A process for the production of grained screens of any required dimensions.
which consists in dusting on to the glass plate a mixture of "dragon's blood" and
asphalte by stirring up the powdered mixture in a closed chamber and in such
manner that the dust must travel upwards so that only the lightest particles can
pass into an adjacent chamber and therein settle upon the plate as herein described.
2. The particular mode of carrying out the process specified in Claim 1, which

page_text: consists in uniformly heating the glass plate whilst exposing it to the vapour
of hydrofluoric acid, so as to prevent condensation of the vapour upon the plate.
3. An apparatus for carrying out the dusting-on process herein specified,
comprising a rotating table upon which the glass plate is supported in the
dusting-on chamber.
4. The special construction of the apparatus specified in Claim 3, characterized
by the employment of a sieve above the plate, the sieve being arranged upon
and rotating with the rotating table substantially as described.
5. An apparatus for carrying out the process specified in Claim 1, characterized
by the arrangement of a generator of hydrofluoric acid vapour in a chamber at
the upper part of which the glass plate to be etched is placed in an inclined
position whilst in an adjacent chamber a current of heated gases is led under the glass
plate support for the purpose of heating the plate and preventing condensation on
its upper surface.
Dated this 29th day of December 1898
A. M. & WM. CLARK,
Chartered Patent Agents, 53, Chancery Lane, London.