AN IMPROVED PROCESS FOR COATING IRON
Year: 1863

page_text: LETTERS PATENT to William Mitchell, of Carlton Hill East, Saint John's
Wood, in the County of Middlesex, Gentleman, for the Invention of
"AN IMPROVED PROCESS FOR COATING IRON."-A communication from
abroad by James Mitchell, residing in New York, United States of
America.
Sealed the 25th September 1863, and dated the 1st April 1863.
PROVISIONAL SPECIFICATION left by the said William Mitchell at the
Office of the Commissioners of Patents, with his Petition, on the
1st April 1863.
I, WILLIAM MITCHELL, of Carlton Hill East, St. John's Wood, in the
County of Middlesex, Gentleman, do hereby declare the nature of the said
Invention for "AN IMPROVED PROCESS FOR COATING IRON," to be as follows:-
The object of this Invention is to apply a coating of copper or other metal,
such as gold, silver, nickel, zinc, tin, antimony, or bismuth, or their alloys, to
the surface of iron. The iron to be coated should be freed from scale and
other impurities in a manner similar to that used for preparing to receive a
coating of tin.
The furnace most suitable for holding a bath of molten metal for covering
sheets of iron is one similar in form to a common puddling furnace; its basin

page_text: should be about twelve inches deep in the centre, running from the centre to
a horizontal line at the surface at an angle of nearly forty degrees. The stack
should be at the opposite end from the fire-house, and the door for introducing
the metal to the bath should be in the centre of the stack, directly under the
flue, so as to overcome the effects of a current of cold air that enters the door.
By experience it has been found that in forming alloys a more solid and per-
fect alloy will be obtained by using the salts of sulphur or arsenic in a modified
form, so as to produce a slight negative action without injuring in any degree
tho tenacity of the alloy, and that alloys of pure metals will more readily com-
bine with iron or with each other through the mineralizing agency of the salts
above mentioned. The furnace being filled with metal to prevent oxidation
and evaporation, the surface is to be covered with a slag formed of sand and
clay mixed to a proper consistency. When the coating of the iron is to be
effected, the slag is pushed back with a ravel some three or four inches, and
the space filled with borax. This will act as a flux for the iron, and keep
the metal in a fluid state. The metal plates or other articles to be coated
having been properly prepared for the bath, are seized hold of with tongs, and
plunged into the bath of metal through the melted borax, and in a short time
raised up and down in the metal bath. When the progress of the amalgama-
tion is perfect, the plate will exhibit a very brilliant luminous appearance.
The plate or other article is then to be withdrawn and handled quickly in such
a manner as to make the fluid metal spread evenly on the surface. The
plate is then laid on a turntable, and the other ond quickly dipped in the
metal bath, using care that the metal coating shall join perfectly in the
centre. A vat of sufficient size lined with sheet lead is provided containing
diluted sulfuric acid, and a nearly saturated solution of oxide of copper, or
the sulphate common blue vitriol of commerce. Into this solution the cleaned
plates or pieces to be coated are immersed, and held there until the surfaco
exhibits a precipitated metallic surface of copper. This covering is a hydrated
compound of the sulphate of iron and metallic copper, and when plunged into
the bath of melted metal, acts as a flux on the iron, and produces immediate
molecular action and separation at a temperature much below a welding heat.
The result is an union of the fluid molecules of both metals, and the negativo
agency produces chemical union. This plate as taken from the first bath, is
again immersed in a saturated solution of alum water, to protect the compound
covering from oxidation while warming before dipping into the bronze bath..

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said William Mitchell in the Great Seal Patent Office on tho
1st October 1868.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, WILLIAM
MITCHELL,. of Carlton Hill East, Saint John's Wood, in the County of
Middlesex, Gentleman, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the First day of April, in the year of our Lord One
thousand eight hundred and sixty-three, in the twenty-sixth year of Her reign,
did, for Herself, Her heirs and successors, give and grant unto me, the said
William Mitchell, Her special license that I, the said William Mitchell, my
executors, administrators, and assigns, or such others as I, the said William
Mitchell, my executors, administrators, and 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 "AN IMPROVED PROCESS FOR COATING
IRON," being a communication to him from abroad by James Mitchell, at
present residing in New York, United States of America, upon the condition
(amongst others) that I, the said William Mitchell, by. an instrument in
writing under my hand and seal, should particularly describe and ascer-
tain the nature of the said Invention, and in what manner the same was to be
performed, and cause the same to bo filed in the Great Seal Patent Office
within six calendar months next and immediately after the date of the said
Letters Patent.
NOW KNOW YE, that I, the said William Mitchell, do hereby declare the
nature of the said Invention, and in what manner the same is to be per-
formed, to be particularly described and ascertained in and by the following
statement (that is to say) :-..
This Invention relates to a novel mode of coating articles of iron with
protecting or lustrous metals, such as copper, gold, silver, nickel, zinc, tin,
antimony, and bismuth, and their alloys.
To enable others skilled in the art of metallurgy to make and use this
Invention, I will proceed to explain the manner of depositing a layer of
copper or a metallic alloy upon the surface of iron.
The iron should be the best charcoal bloom, sound and free from blister, and
of uniform temper for making what I shall denominate bronze iron; for spikes,

page_text: bolts, bars, braces, bands, or other appliances for ship-building, the iron should
be good common sound iron. In preparing the iron for being rolled into
sheets, care must be taken to obtain a surface free from indentation caused by
scales of oxide being forced into the plates of iron in the process of rolling.
The oxide scale should be removed in time, so as to obtain a smooth surface
'similar to what is required for receiving a coat of tin. For more particular
work the iron should be rolled both ways to produce a cross fibre. The size
of the plates when bronzed should always correspond to the size required when
finished, keeping in view the increase in length by rolling. For all other
purposes the iron to receive the coat of metal should be prepared in the form
and size required for use.
For removing scales or oxide from the surface of the metal articles to be
coated, so as to present a clean bright surface, I immerse the articles in a bath
of diluted sulphuric acid contained in a vat of a suitable size, and lined with
sheet lead. When the articles have remained a sufficient time in the bath to
loosen the scales, I withdraw them, and subject them to a scouring process
to produce a bright surface. The articles are then to be washed to free them
from the acid, and they may then bo deposited in a vat containing an alkaline
preparation to prevent rusting. The use of lime should be avoided, as it will
deposit an insoluble sulphate that will prevent the bronze from combining with
the iron; soda ash or potash is the best for forming the alkaline bath. The
articles may remain in this bath until wanted for dipping in the bronze.
The furnace most suitable for holding a bath of molten metal for covering
sheets of iron or other articles of iron prepared in the manner hereafter
described, is similar in form to a common puddling furnace. Its length and
breadth should be in proportion to the quantity of metal it is desired to contain,
having respect also to the size of the sheets to be bronzed, its basin should
be about twelve inches deep in the centre, running from the centre to a hori-
zontal line at the surface at an angle of nearly forty degrees. The chimney
stack should be at the opposite end from the fire-place, and the door for intro-
ducing the metal to the bath should be in the centre of the stack, directly
under the fluc, so as to overcome the effects of a current of cold air that enters
the door. The current of hot air meeting the current of cold air, both pass up
the flue, and prevent the chilling effects that would otherwise be produced
upon the metal. The crown of the furnace should correspond in shape to the
form of the basin, so as to force the flame into the bottom of the basin to insure
heat. The best materials for the furnace are first-class solid fire-brick, set in
grout composed of fire-clay and plumbago. The form of the brick for the
bottom should be that of a wedge with the thick end down to prevent the

page_text: bottom from rising up. The sides should be of the same materials, straight
brick laid in grout..
I will now consider the composition of the bath of metal, copper alone, or
alloyed (to suit the fancy as to substance) with any of the metals abovo
described. It has been found by experience that in forming alloys a more
solid and perfect alloy will be obtained by using the salts of sulphur or arsenic
in a modified form, so as to produce a slight negative action without injuring
in any degree the tenacity of the alloy, and that pure metals will more readily
combine with iron or with each other through the agency of the salts above
mentioned. The furnace is to be filled with such metal as it is desired to
employ to a level with the bottom of the door sill; to make it convenient for
use, and to prevent oxidation and evorporation, the surface must be covered
with a slag for its protection formed of sand and clay mixed to a proper
consistency. When about to use the bath, the slag is to be pushed back with
a ravel some three or four inches, and the space filled with borax. This will
aid as a flux for the iron, and keep the metal in a fluid state, thereby insuring
a smooth surface for the coated article of iron.
The manner of operating with the prepared iron plates in the bath is as
follows: Seize hold of the plate with suitable tongs, and plunge it into tho
bath of metal through the melted borax to the depth of more than half its
length, and in a short time raise it up and down in the metal bath, and observe
the progress of the amalgamation until it is perfect, when it will exhibit a very
brilliant luminous appearance; then withdraw the plate, handling it quickly, in
such a manner as to make the metal spread evenly on the surface, hold it up,
and give the fluid metal a chance to retain an even thickness in the centre of
the plate; then lay it on a turntable, and quickly dip the other end of the
plate in the metal bath, using care that the metallic coating shall join perfectly
in the centre.
I have described the quality of iron requisite for a good product, and the
method of cleaning and preparing the same for use, the form of the furnace
and the manner of building the same, the kind of metals required for the bath,
and the manner of operating with metal plates to effect the coating or bronzing
of the same, and I will now explain the mode of preparing the cleaned plates
or other articles for immersion in the molten metal.
A second vat, lined with sheet lead, and of sufficient size to admit of the
complete immersion of the metal plates or other articles, is provided to contain a
bath composed of diluted sulphuric acid, and oxide of copper, or the sulphate of
copper (common blue vitriol of commerce). In this solution the plates or other
articles to be bronzed or coated aro immersed, and retained until their surface

page_text: .exhibits a precipitated metallic surface of copper. This covering is a hydrated
compound of sulphate of iron and metallic copper, which, when plunged into
the bath of molten metal, acts as a flux on the iron, and produces immediate
molecular action and separation at a temperature much below a welding heat,
the result being an union of the fluid molecules of both metals. The coated
iron articles, as taken from the oxide of copper or sulphate of copper bath, are
next immersed in a saturated solution of alum, to protect the compound
covering from oxidation while warming before dipping into the bronzo or
molten metal bath.
In rolling sheet iron bronzed or coated according to the above-described
process, great care should be taken in heating them if a fire with a current of
air is used, inasmuch as the bronze coating will be easily oxidized and greatly
injured thereby. A close muffle, with a wood or charcoal fire, is preferred for
safety, and to insure success.
These bronze iron sheets will be found useful in many appliances where
slicet copper, iron, or brass is now used, in all parts of the construction of ships
and other water craft.
Having now set forth the nature of the improved process for coating iron,
and explained the manner of carrying the same into effect, I wish it to be
understood that under the above in part recited Letters Patent I claim, the
means above described for effecting an intimate combination of the covered
and covering metals, whereby all tendency to galvanic action between the iron
.and the covering metal or alloy will be removed.
In witness whereof, I, the said William Mitchell, have hereunto set my
hand and seal, the First day of October, in the year of our Lord One
thousand eight hundred and sixty-three.
Witness,
J. W. MOFFATT,
66, Chancery Lane.
W. MITCHELL. (L.S.)