Improvements in the Manufacture of Mirrors.
Year: 1893

page_text: COMPLETE SPECIFICATION.
[Communicated from abroad by JOHN JACOBSON, of Boston, County of Suffolk
and State of Massachusetts, United States of America, Artist.]
Improvements in the Manufacture of Mirrors.
I, HENRY HARRIS LAKE, of the Firm of Haseitine, Lake & Co., Patent Agents,
45 Southampton Buildings, in the County of Middlesex, do hereby declare the
nature of this invention, and in what manner the same is to be performed, to be
particularly described and ascertained, in and by the following statement:-
This invention relates to a metallic surfaced mirror or speculum and to the
method of making the same, the object being to produce at low cost a mirror
having a suitable surface for optical purposes.
In a former Patent No. 22730 dated December 29th 1891 I have shown and
described a method of making mirrors in which a metallic film is deposited upon a
form having a suitable polished surface converse in shape to that required for the
mirror (that is, the surface of the form being concave if the mirror is to be convex,
and rice versa) and a backing composed, in part at least, of plastic material is
applied to the metallic film while on the form, and the said backing with the film
adhering to it is subsequently removed from the form so that the surface of the
film that was in contact with the form when deposited thereon becomes the exposed
reflecting surface of the mirror.
The present invention consists in a specific method of carrying out the plan of
operation described in said former Patent and in a specific construction of the
resulting mirror which method and mirror has been invented by me as the result of
long experimenting for the purpose of discovering a suitable material or combination
of materials to be used in making the mirrors.
In my former Patent I have described gum shellac as the best material known to
me at that time to be used as a plastic cement to constitute in whole or in part the
backing of the metallic film but the said material has some objections such as
liability to contract or to change its shape in the course of time, under the conditions
to which the mirror is subjected in use.
The method forming the subject of the present invention consists in the following
steps: First depositing upon a properly polished and finished mold a film of
metal; second, applying a back of plaster of paris to said film while on the mold,
said plaster of paris being applied while in the semi-fluid condition requisite for
making plaster casts, and being permitted to set, harden, and dry upon the film;
third, completely filling the pores of the said plaster of paris, so as to expel the
air therefrom, by a suitable material such for example as paraffine or mineral
wax and finally removing the film with its backing of plaster of paris from
the form and thus obtaining the mirror forming the subject of this invention,
composed of a deposited reflecting metallic surface film and a base of plaster of
paris, having its pores filled with paraffine or analagous wax-like material.
If the air is not expelled from the pores of the plaster backing, the latter with
the metallic film cannot be effectively removed from the form as the film is likely
to adhere as strongly to the form as to the backing and to thus become torn or to
wholly fail to be removed from the form; and if it should be removed it would still
be likely to break away from the plaster in places and thus destroy the perfect
optical shape and condition of the surface.
By excluding the air from the pores of the plaster, however, these objections are
overcome and a permanent and durable mirror is produced which is not liable to
destruction under the conditions of use and which will not deteriorate in any way
under ordinary conditions.

page_text: Instead of saturating the plaster of paris with paraffine for the purpose of
excluding the air or filling its pores the plaster of paris may be rendered non-porous
as follows: After the plaster is thoroughly set and dried it is saturated with
alcohol and is then covered to a considerable depth with a solution of shellac in
alcohol and is gently heated.
By this method the shellac finds its way into the pores of the plaster of paris
filling the same so as to exclude the air and render the plaster of paris sufficiently
adhesive to the metallic film to enable the latter to be detached with the plaster
froin the form on which it is deposited. This mode of treatment requires more
time than the saturation with paraffine but renders the plaster of paris backing
harder than when paraffine is used, and for many purposes is the preferable mode
of filling the pores of the plaster.
The preliminary saturation of the plaster in alcohol ensures the complete
permeation of the shellac solution throughout the body of the plaster to the
metallic film in contact therewith.
In the accompanying drawing,
Figure represents the form a, with the metallic film b, deposited thereon,
said form being provided with a peripheral flange or rim a², which may be
employed to contain the solution from which the metal is deposited, and also
to produce a suitable mold to contain the plaster of paris when applied in plastic
condition.
Figure 2 represents the mold and film a, and b, as shown in Figure 1, and the
backing, of plaster of paris, set and hardened on the said film.
Figure 3 represents the same parts as shown in Figure 2, but with the pores of
the plaster of paris filled with wax-like material, such for example as paraffine or
shellac, as indicated at d, to exclude the air from the pores and thus ensure perfect
adhesion between the plaster and metallic film, and
Figure 4 represents the completed mirror, all the said figures being shown in
section transverse to the form and mirror.
Having now particularly described and ascertained the nature of the said
invention and in what manner the same is to be performed, as communicated to me
by my foreign correspondent, 1 declare that what I claim is :-
1. As an improved article of manufacture the herein described mirror, composed
of a deposited metallic reflecting surface, a base of plaster of paris cast thereon,
and an air expelling filling in the pores of the said plaster base, substantially as set
forth.
2. The improvement in the art of making metallic mirrors which consists in
depositing upon a suitably shaped and finished mold or form a film of metal whose
reflecting surface next to the mold is completed and finished by deposition thereon;
then applying a backing of plaster of paris to the film while on the mold, and
permitting the same to set and harden, then filling the pores of the said plaster of
paris backing and finally removing the said backing with the film adhering thereto
from the mold, substantially as described.
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, W.C., Agents for the Applicant.