IMPROVEMENTS IN THE PREPARATION OF MIRROR PLATES OR LOOKING- GLASSES.
Year: 1875

page_text: LETTERS PATENT to John Henry Johnson, of 47, Lincoln's Inn
Fields, in the County of Middlesex, Gentleman, for the Invention
of "IMPROVEMENTS IN THE PREPARATION OF MIRROR PLATES OR LOOKING-
GLASSES.". A communication from abroad by Jean Joseph Etienne
Lenoir, of Paris, in the Republic of France, Engineer.
Scaled the 9th November 1875, and dated the 31st May 1875.
PROVISIONAL SPECIFICATION left by the said John Henry Johnson
at the Office of the Commissioners of Patents, with his Petition, on
the 31st May 1875.
I, JOHN HENRY JOHNSON, of 47, Lincoln's Inn Fields, in the County
of Middlesex, Gentleman, do hereby declare the nature of the said
Invention for "IMPROVEMENTS IN THE PREPARATION OF MIRROR PLATES OR
LOOKING-GLASSES," a communication to me from abroad by Jean Joseph
Etienne Lenoir, of Paris, in the Republic of France, Engineer, to be
as follows:-

page_text: whiteness or lustre to such glasses equal to that obtained in glasses pre-
pared with quicksilver.
The Invention consists of means of causing mercury dissolved in an
acid or in an alkali, or both, to penetrate into the metal deposited upon
the mirror plate so as to whiten and impart lustre to mirrors or glasses
when plated with silver and the like.
When by means of a chemical reaction a white metal, such, for
example, as silver is deposited upon a mirror plate or other vitrified
body, the metal forming the base of the mirror reflects the image well,
but it is dull and usually of a yellow or darkish tint.
1
According to this Invention in order to effect the above object and
obviate the inconvenience before mentioned, the process of preparing the
glass is carried out in the following manner:-After having deposited
silver, platinum, tin, bismuth, or other suitable white metal upon a
mirror plate, the metal is coated with a varnish of gum lac, after which ]
the mirror plate is laid flat or in a horizontal position and covered for a
period of say about half an hour with a layer of a solution composed in
the following manner :-In about one quart of water there is dissolved
about 3 oz. of cyanide of potassium, after which nitrate of mercury is
added until saturation, that is to say, until a slight precipitate is 2
formed. To this solution there is added acetic acid until the liquid
imparts a red tint to litmus paper, after which the solution is further
diluted with about 5 quarts of water so as to produce about 6 quarts of
1
1

page_text: say about eight parts of water and laid on the mirror plate with a brush
before varnishing.
When the mirror is well brightened it should be washed with water,
dried, and then painted, or have tin foil applied thereto. By these means
the mirror has imparted to it a lustre or brightness equal to that
obtained with a quicksilver mirror and not liable to deteriorate.
SPECIFICATION in pursuance of the conditions of the Letters Patent
filed by the said John Henry Johnson in the Great Seal Patent
Office on the 30th November 1875.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JOHN
HENRY JOHNSON, of 47, Lincoln's Inn Fields, in the County of
Middlesex, Gentleman, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Thirty-first day of May, in the year
of our Lord One thousand eight hundred and seventy-five, in the
38th year of Her reign, did, for Herself, Her heirs and successors,
give and grant unto me, the said John Henry Johnson, Her special
license that I, the said John Henry Johnson, my executors, admini-
strators, and assigns, or such others as I, the said John Henry Johnson,
my executors, administrators, or assigns, should at any time agree
with, and no others, from time to time and at all times thereafter
during the term therein expressed, should and lawfully might make,
use, exercise, and vend, within the United Kingdom of Great Britain
and Ireland, the Channel Islands, and Isle of Man, an Invention
for "IMPROVEMENTS IN THE PREPARATION OF MIRROR PLATES OR LOOKING-
GLASSES," a communication to me from abroad by Jean Joseph Etienne
Lenoir, of Paris, in the Republic of France, Engineer, upon the con-
dition (amongst others) that I, the said John Henry Johnson, my
executors or administrators, by an instrument in writing under my
hand and seal, should particularly describe and ascertain the nature
of the said Invention, and in what manner the same was to be
performed, and cause the same to be filed in the Great Seal Patent
Office within six calendar months next and immediately after the date
of the said Letters Patent.
}

page_text: NOW KNOW YE, that I; the said John Henry Johnson, do hereby
declare the nature of the said Invention, and in what manner the same
is to be performed, to be particularly described and ascertained in and
by the following statement:-
Since the introduction of the new process for silvering mirrors by
chemical means, the old process of coating them with amalgamated tin
foil has fallen into disuse. The process was unhealthy to the workmen,
more expensive in its application, and the tin amalgain was liable to
liquate and become dull or cracked. On the other hand, however, the
present system of silvering by chemical means does not produce mirrors
possessing all the brightness eclat of those produced by tin amal-
gamation. The silver film has always a more or less yellowish tinge,
and the coloration is liable rather to increase with age.
The object of this Invention is to amalgamate the silver film by
chemical means, and I thus produce more perfect adherence of the
silver to the glass and notably increase the brilliancy of the mirror
surface and the purity of its colour.
The process is as follows:-The plate of glass is coated with silver by
any one of the known methods. This being done the surface may be
washed free from all deposit by being flushed with water. I now pour on
the silvered surface a solution of a salt of mercury. Many of the salts
of mercury answer more or less. Thus, oxide of mercury, oxalate,
chloride, subchloride, and many others will answer if they are dissolved
in a solution which is of a nature to clean and dissolve away from the
silver surface the deposit produced by the decomposition of the mer-
curial salt; but of all solutions that which I prefer is one where a salt
of mercury is dissolved in cyanide of potassium, and notably where a
double cyanide of mercury and potassium has been formed by dissolving
cyanide of mercury in cyanide of potassium.

page_text: surfaces by the application of solutions of mercurial salts with a view
to improving the colour and increasing the durability of such surfaces.
In witness whereof, I, the said John Henry Johnson, have
to this my Specification set my hand and seal, this Thirtieth
day of November, One thousand eight hundred and seventy-
five.