Improvements in the Production of Coatings or Films on Glass and other Articles or Materials, and in Apparatus therefor.
Year: 1897

page_text: Date of Application, 14th May, 1897
Complete Specification Left, 14th Feb., 1898-Accepted, 14th June, 1898
Improvements in the Production of Coatings or Films on Glass and
other Articles or Materials, and in Apparatus therefor.
I, JAMES WHEELER, of 100, High Street, Ilfracombe, in the County of Devon,
Pharmaceutical Chemist, do hereby declare the nature of this invention to be as
follows:-
This invention relates to the production on articles of films containing betulin
or a compound thereof.
According thereto for this purpose articles on which films are to be produced
are exposed in a closed chamber to fumes produced by burning, without flame,
a combustible material comprising or containing the epidermis or outer rind of
birch bark to an amount bearing a suitable proportion to the area of the chamber.
According to the conditions under which the combustible material is burnt there
is produced (a) an opaque film that is pervious to hydrofluoric acid so that, when
produced on suitable glass and subjected to the action of that acid, it will enable
the said acid to act upon the glass in such a manner as to impart to it a finely pitted
translucent surface such as to render it suitable for use as focussing-glass or other
purposes; or (b) a transparent film that offers great resistance to hydrofluoric acid
and also to dampness and the attack of insects or fungoid growths. The opaque
and pervious film I consider to consist wholly or chiefly of pyrobetulin and the
transparent of pyrobetulin anhydride.
In carrying my invention into practical effect I may employ the combustible
material in the form of tablets made by compressing together a mixture comprising
finely ground epidermis of birch bark-preferably that of white birch or betula
alba and an oxygen-carrier such as nitrate of potassium. The percentage of
nitrate of potassium in the material may vary; for the production of films for
etching purposes there may be for example about 8 per cent. of nitrate of potassium
and when intended for producing films for protecting articles from damp, insect-
attack, etc., the percentage of nitrate of potassium may be from 2 to 4. In order
to produce an opaque film, the tablets are arranged so as to give an air-space
around each, whereas to produce a transparent film the tablets are placed close
together, whereby a higher temperature of combustion will be attained than
with the former arrangement. The fumes are preferably filtered before being
deposited, and in producing opaque films it is desirable to agitate the fumes given
off in such a manner as to ensure uniform deposition.
Apparatus for filming articles according to this invention may comprise a
filming chamber in which the articles to be filmed are placed and which may
have in its top an opening through which it communicates with an upper com-
partment formed by a conical cover that fits upon the top of the chamber, forming
therewith around the opening an air-tight joint by means of an india rubber
ring and a flange or other suitable means. At the centre of the said opening
there is supported from the top of the chamber, a suitable stand for a lamp in
which to burn the tablets. This lamp may comprise a cylindrical cup perforated
го

page_text: at its lower part to admit air and adapted in its upper part to support a floor-plate
on which is placed a ring or distance-piece which supports a charge-plate upon
which the tablets are arranged with their lower edges resting more or less on
the floor-plate. To produce opaque films the floor-plate may advantageously con-
sist of two concentric rings of metal connected by radial pieces of metal and the
charge-plate may be a plate of metal having a number of radial openings made
through it for the due retention of the tablets with an air space around each, each
opening widening out towards the periphery. For producing transparent films,
the floor plate may advantageously consist of a metal ring having opposite sides
connected together by a number of parallel bars and the charge-plate may consist
of a similar ring having two intersecting sets of bars that connect opposite sides
but are placed at a comparatively great distance apart and between which the
tablets are arranged in close contact transversely to the bars of the floor-plate.
The lamp is provided with chimney of wire gauze, which is adapted to be supported
on the upper end of the cup and to project upwards therefrom and which is sur-
rounded by and covered with filtering material such as muslin. Within the conical
cover, which has a domed top, there is arranged a metal cylinder and within this
cylinder there is mounted a suitable apparatus, such for example as a vertical
egg-whisk for agitating the internal atmosphere, this apparatus being connected
by suitable gearing to a crank-handle or other suitable operating means outside
the cover and the metal cylinder being provided below the agitating-apparatus
with a conical guide or deflector. In using this apparatus, the articles to be
filmed having been placed in the filming-chamber and the tablets having been
suitably arranged in the lamp, the latter are ignited simultaneously by means of
a jet of gas caused to play upon the upper surfaces thereof, and, any flame produced
having been extinguished, the chimney is placed upon the cup and the lamp
put upon its stand. The conical cover is then placed in position and the agitating
apparatus put into motion. The fumes, on being formed, rise upwards through
the cylinder and are deflected by the guide towards the centre of the agitating
apparatus, through which they pass to the top of the cover, which deflects them
downwards and causes them to descend between the cover and the metal cylinder
into the filming-chamber, with the result that, having been thus intimately
mixed together and with the air within the cover, they form uniform films upon
the articles in the filming-chamber.
For producing an opaque film on glass I have obtained satisfactory results with
an apparatus as above described in which the filming-chamber had a
length of
22 inches, a breadth of 16 inches, and a depth of 8 inches, whilst the opening in
its top was 9 inches in diameter, the height of the cover 12 inches, and the diameter
of the cup of the lamp 2 inches, this being adapted for tablets of about 8 inch
in diameter. I have for this purpose and with this apparatus employed tablets
weighing 50 grains i.e., in the ratio of 20 grains to each superficial foot of the
area of the filming-chamber. With this apparatus I have found it suitable to
operate the agitating-apparatus for fifteen minutes and then to allow a further
fifteen minutes for the due subsidence of the fumes. I have then removed the
plates of glass from the chamber and exposed them to the air, or in a drying-
chamber, to mature the films, for not less than three days. In producing a trans-
parent film on glass or other material it is unnecessary to employ the agitating-
-apparatus, and this apparatus-is,_before_doing this, preferably removed.
In filming a tube and like article, it may advantageously be mounted on a core
and rotated whilst the deposit is being formed.
Films produced in this way may be employed for various purposes in addition
to those hereinbefore referred to. For example, it has been found that lint filmed
according to this invention acts very beneficially as a preventative to the formation
of pus when applied to wounds. Wound-dressing materials other than lint may,
of course, be similarly treated with a like result. Again, films produced according
to this invention on paper impart to it a surface admirably adapted for receiving
impressions of engravings etc.; and in order to vary the natural india-tint of the

page_text: film when used for this purpose, an addition of from 5 to 10 per cent. of asphaltum,
dragon's blood, or other suitable sublimable material may be made in the compo-
sition of the tablets, the quantity of nitrate of potassium being proportionately
increased adequately to effect their proper combustion. By filming finished photo-
graphs according to this invention, fine effects of softness and heightening of
shadows can be produced. The tin amalgam or other silvering of mirrors, moreover,
may be filmed in accordance with this invention to protect it from cracking
and becoming "fogged" by contact with a damp wall.
For reducing the epidermic of birch bark to a sufficiently fine powder to produce
the combustible material or tablets, I first pass it through a disintegrator until
it is reduced to a finely shredded condition; I then dry it by exposing it to the
air freely and to a temperature of 120° F. during three days; and I then pass
it repeatedly under heavy stone edge-runners until sufficiently fine to pass through
a sieve having sixty holes to the linear inch.
Improvements in the Production of Coatings or Films on Glass and
other Articles or Materials, and in Apparatus therefor.
I, JAMES WHEELER, of 100, High Street, Ilfracombe, in the County of Devon,
Pharmaceutical Chemist, do hereby declare the nature of this invention and in
what manner the same is to be performed to be particularly described and ascer-
tained in and by the following statement:-
This invention relates to the production on articles or materials of films or
coatings containing betulin or betulin anhydride.
According thereto for this purpose articles on which films or coatings (hereinafter
called films) are to be produced are exposed in a closed chamber to fumes produced
by burning, without flame, a combustible material comprising or containing the
epidermis or outer rind of birch bark to an amount bearing a suitable proportion
to the area of the chamber. According to the conditions under which the com-
bustible material is burnt there is produced (a) a porous opaque film that is pervious
to hydrofluoric acid so that, when produced on suitable glass and subjected to the
action of that acid, it will enable the said acid to act upon the glass in such a
manner as to impart to it a finely pitted translucent surface such as to render it
suitable for use as focussing-glass or other purposes requiring a large amount of
highly diffused light; or (b) a transparent film that, according to the composition
of the combustible material employed for its production, either has an even surface
is non-porous, and is adapted to offer great resistance both to hydrofluoric acid
and also to dampness and the attack of insects or fungoid growths, or, when
deposited upon a smooth non-absorbent surface, presents a reticulated appearance
and to an extent dependent upon this appearance is readily pervious to hydro-
fluoric acid so that, when subjected to the action of that acid, it will impart a
similar reticulated appearance to suitable glass on which it has been deposited.
The porous opaque and the non-porous and reticulated transparent films are simi-
larly pervious or impervious to the action of agents, such as a solution of nitric,
hydrochloric or sulphuric acid, suitable for etching polished metal surfaces. The
porous opaque film consists chiefly of pyrobetulin and the non-porous and reticu-

page_text: lated transparent films of pyrobetulin anhydride-that is to say, respectively of
betulin and betulin anhydride produced by burning and consequent dry distillation
instead of by the action of a solvent, such as boiling alcohol, which has hitherto
been the means employed for obtaining betulin.
In carrying my invention into practical effect I employ the combustible material
in the form of tablets or small blocks (hereinafter called tablets) made by com-
pressing together a mixture comprising finely ground epidermis of birch bark-
preferably that of white birch or betula alba-and an oxygen-carrier such as nitrate
of potassium. The percentage of nitrate of potassium in the material may vary:
. for the production of porous opaque or non-porous transparent films for etching
glass or metal there may be, for example, about 8 per cent. of nitrate of potassium ;
and for producing transparent films for protecting articles from damp, insect-
attack, etc., the percentage of nitrate of potassium may be from 1 to 4; reticulated
transparent films are produced on smooth non-absorbent surfaces from mixtures
having low percentages of nitrate of potassium such as 1 or 2: when less than
per cent. of nitrate of potassium is used the mixture should be dried for a con-
siderable time both before and after compression. In order to produce a porous
opaque film, the tablets are arranged so as to give an air-space around each,
whereas to produce a non-porous or reticulated transparent film the tablets are
placed close together, whereby a slower combustion will be attained than with
the former arrangement: the result of the respective arrangements is that whereas
in the one case pyrobetulin is obtained by the burning away of the cortical invest-
ment of the betulin of the bark, in the other case this pyrobetulin is more or less
completely converted into its anhydride by the more prolonged heating and slower
evolution of the fumes..
The fumes are preferably filtered before being deposited, and in producing porous
opaque films it is desirable to agitate the fumes given off in such a manner as
to ensure uniform deposition.
Figs. 1 and 2 of the accompanying illustrative drawings are sectional views,
taken at right angles to each other, of an apparatus suitable for filming various
articles and materials according to this invention;
Fig. 3 is a partial plan representing a support for a lamp;
Fig. 4 is a detail-view showing in elevation a portion of a support for the
agitating-apparatus;
Fig. 5 shows the lamp half in vertical section and half in elevation and provided
with Hoor and charge plates for producing porous opaque films;
Figs. 6, 7, and 8 are respectively a plan of a charge-plate, an elevation of an
annular distance-piece, and a plan of a floor-plate used in producing porous opaque
films; and
Figs. 9 and 10 are respectively plans of a charge-plate and a floor-plate used in
producing non-porous and reticulated transparent films.
Figs. 5 to 10 inclusive are drawn to a larger scale than the other figures.
The apparatus shown comprises a filming-chamber 1 of sheet zinc in which the
articles to be filmed are placed and which has in its top a circular opening 2
through which it communicates with an upper compartment 3 formed by a conical
cover 4 that fits upon the top of the chamber 1, forming therewith around the
opening 2 an air-tight joint by means of an india-rubber ring 5 and a flange G.
The bottom of the chamber I may be closed in a sufficiently air-tight manner
by placing it upon a thick cloth or other suitable compressible material. At the
centre of the opening 2 there is supported from the top of the chamber 1, by means
of bars 7 (see Fig. 3) soldered or otherwise suitably secured thereto, a suitable
stand 8 for a lamp for burning the tablets. This lamp may, as shown, comprise
a cylindrical cup 9 (Fig. 5) perforated (as shown) at its lower part to admit air
and adapted in its upper part to support, by means of brackets 10, a floor-plate 11
on which is placed a ring or distance-piece 12 (Fig. 7), which supports a charge-
plate 13 in which the tablets are arranged with their lower edges resting more
or less on the floor-plate 11. To produce porous opaque films the floor-plate may

page_text: For
advantageously consist, as shown in Fig. 8, of two concentric rings 14 and 15 of
metal connected by radial pieces 16 of metal, and the charge-plate 13 may, as
shown in Fig. 6, be a plate of metal having a number of radial opening 17 made
through it for the due retention within them of the tablets with an air-space around
each tablet (17" are air-holes), the edges of the tablets will then rest more or less
on the ring 14. It is obvious that for a larger lamp the floor-plate may have two
or more concentric rings 14 and the charge-plate a corresponding number of con-
centric rings of tablet containing holes 17 with intervening air-holes 17".
producing non-porous or reticulated transparent films, the floor-plate may advan-
tageously consist, as shown in Fig. 10, of a metal ring 18 having opposite sides
connected together by a number of parallel bars 19, and the charge-plate may
consist, as shown in Fig. 9, of a similar ring 20 having two intersecting sets of
bars 21 and 22 respectively, that connect opposite sides but are placed at a com-
paratively great distance apart and between which the tablets are arranged close
together transversely to the bars 19 of the floor-plate. The lamp is provided with
a removable chimney 23 of wire gauze, which rests on and within a collar 24
secured near the upper end of the cup 9 and is more especially for the preparation
of the porous opaque films, surrounded by, and covered with, filtering material 25
such as muslin. Besides acting as a support to the chimney the collar 24 serves
to prevent, air from entering the chimney between the latter and the cup 9, to an
injurious degree. Within the conical cover 4, which has a domed top 26, there
is arranged a metal cylinder 27 supported by a cross bar 28, which engages in
slots 29 in the cylinder 27 and is secured at its ends to a conical ring 30 removably
connected to the cover 4 by bayonet-joints consisting of pins 31 projecting from
the cover 4, and slots 32 (see Fig. 4) in the said ring. Within the cylinder 27
there is mounted a suitable apparatus, such for example as a vertical egg-whisk 33,
for agitating the internal atmosphere, this apparatus having a crank-handle 34
or other suitable operating means outside the cover 4 and the metal cylinder 27
being provided below the agitating-apparatus with a conical guide or deflector 35.
In using this apparatus, articles to be filmed are placed in the filming-chamber 1
care being taken that the articles do not overlap one another, and have nothing
above them in the chamber, since the fumes descend in the manner of dew; the
tablets, having been suitably arranged in the lamp, are ignited simultaneously
by means of a jet or jets of gas caused to play upon their upper surfaces, and,
any flame produced having been immediately extinguished, the chimney 23 is
placed upon the cup 9 and the lamp put upon its stand 8. The conical cover 4
is then placed in position and the agitating-apparatus 33 put into motion. The
fumes, on being formed, rise upwards through the cylinder 27 and are deflected
by the guide 35 towards the centre of the agitating-apparatus 33, through which
they pass to the top 26 of the cover 4, which deflects them downwards and causes
them to descend between the cover 4 and the metal cylinder 27 into the filming-
chamber 1, with the result that, having been thus intimately mixed together and
with the air within the cover 4, they form uniform films upon the articles in the
filming-chamber. The annular recess formed by and between the cylinder. 27
and the external surface of the deflector 35 serves to catch carbonaceous matter
which collects upon the agitating-apparatus 33, and, especially if the latter is
not occasionally cleaned, is thrown from it to the sides of the cylinder in very thin
flakes, which, if allowed to fall upon the films, would leave corresponding marks
upon them when finished. It is advisable that, especially in cold weather, the
lamp and its charge should, when arranged for producing transparent films, be
preparatorily heated as much as is consistent with its proper handling in order
more completely to prevent at the outset the evolution of low temperature fumes
adapted to produce porous opaque films. 36 and 37 are apertures provided at
opposite ends of the chamber 1 and normally closed by screw-plugs, as shown:
after a charge has been burnt and before igniting a new one, the conical cover 4
is removed and fresh air is pumped or blown through the apertures 36 and 37
to displace the air that is no longer capable of supporting combustion.

page_text: For producing a porous opaque film on glass or metal I have obtained satis-
factory results with an apparatus as above described in which the filming-chamber
had a length of 22 inches, a breadth of 16 inches, and a depth of 8 inches, whilst
the opening in its top was 9 inches in diameter, the height of the cover 12 inches,
and the diameter of the cup of the lamp 24 inches, this being adapted for small
tablets of about 5 inch in diameter. I have for this purpose and with this
apparatus employed tablets weighing collectively 50 grains-i.e., in the ratio
of 20 grains to each square foot of the area of the filming-chamber. With this
apparatus I have found it suitable to operate the agitating-apparatus for fifteen
minutes and then to allow a further fifteen minutes for the due subsidence of the
fumes. I have then removed the plates of glass or metal from the chamber and
exposed them in the open air, or preferably in a drying-chamber, to mature the
films, for not less than three days. In producing non-porous and reticulated
transparent films on glass or metal and when filming other material it is unneces-
sary to employ the agitating-apparatus and this apparatus together with the
cylinder 27 is, before doing this, preferably removed. For producing on glass
or metal for etching-purposes non-porous and reticulated films, suitable charges
weigh from 40 to 60 grains and 40 grains respectively to the square foot of the
area of the filming-chamber. For filming-chambers of larger area than that of
which the dimensions have been given, whilst the proportions of the charges for
non-porous transparent films may remain the same, that for porous opaque films
should be reduced-thus for a chamber of an area of 9 square feet a suitable
charge is one of two drachms; the charges for reticulated films should be so regu-
lated that after the contraction of the film causing the reticulated appearance
there would be no further deposition-when a reticulated film has been properly
deposited upon a plate of glass and matured the glint of the glass is visible through
the pervious portions of the film, whereas if there be further deposition after con-
traction this will not be the case. Porous opaque films should not be thicker
than necessary, for the thicker they are of course the longer must be the exposure
to the action of the etching-fluid, and even with the longer exposure, the result
is less satisfactory than with films of proper thickness: a safe rule to follow, as
regards glass, (and as the same thickness of film is correct for metals, a trial-slip
of glass may be filmed along with metal articles) is that on removing a porous
opaque film on glass from the filming-chamber immediately after the film has
been deposited, a one-inch jet of gas should be seen clearly outlined through the
filmed plate when the plate is held at a distance of three feet from the jet. The
grain of reticulated films varies with the temperature of the air in the apparatus
in which the filming is performed--for instance, the grain of a film produced in
an unwarmed chamber would be finer in cold weather than in hot summer weather;
the grain may also be made finer by reducing the charge below that of 40 grains
per square foot hereinbefore mentioned. To insure uniformity in the size of the
grain of a reticulated film all that is necessary is to equalize the temperature
within the filming chamber by some suitable means: in the case of the apparatus
of which the dimensions are hereinbefore given, hot bricks placed in close contact
with the two ends of the chamber are adequate for the purpose; chambers of larger
area may each either be provided with a jacket between, which and the chamber
warmed air will be caused to pass, or have a lining of asbestos to prevent radiation
of heat. Reticulated films are preferably kept for 2 or 3 days in a dry atmosphere
before being subjected to the action of an etching-fluid, which should be a weak
one in the case of glass the fuming aqueous hydrofluoric acid of commerce
mixed with not less than 50 per cent. of water is suitable. Ordinary glass etched
by means of a reticulated film has imparted to it a surface of like reticulated
appearance, which whilst offering no increased resistance to the passage of light,
is yet sufficiently opaque for all purposes where obscurity to vision alone is required.
Flashed glass treated in this way gives rise to brilliant effects of scintillation of
light which render it well suited for use, as cathedral glass.
In the case of a filming chamber of very large area it may for the preparation

page_text: of porous opaque films be necessary to provide a fan, which may be arranged to
revolve in a circle slightly larger than the opening 2 and be slowly revolved by
a motor, which would also work the agitating-apparatus. Also, further to facili-
tate the working, a hook or ring may be fitted at each corner and connected with
a common rope or chain, passing over an overhead pulley for raising the chamber
to allow of the removal of the filmed material therefrom. Furthermore the conical
cover may be made in two parts forming an air-tight joint with each other by
means of flanges, the lower part supporting the stand for the lamp and having
one or more internal annular grooved depressions or channels adapted to prevent
condensed moisture from running down and falling upon the films.
In filming a tube or like article, it may advantageously be mounted on a core
and rotated whilst the deposit is being formed.
Films may be produced on powders in this manner. If a powder be lightly
sifted over the floor of the filming chamber and a film produced upon it as herein-
before described, an intimate mixture of the same (whether consisting of pyro-
betulin or of pyrobetulin anhydride) with the powder can be easily effected
immediately after the deposition of the film.
For reducing the epidermis of birch bark to a sufficiently fine powder to produce
the combustible material or tablets, I first pass it through a disintegrator until
it is reduced to a finely shredded condition; I then dry it by exposing it to the
air freely and to a temperature of 120° F. during three days; and I then pass
it repeatedly under heavy stone edge-runners until sufficiently fine to pass through
a sieve having 60 holes to the linear inch.
Transparent films formed from a mixture containing about 2 per cent. of nitrate
of potassium and produced in the manner described may be employed for various
purposes in addition to those herein before referred to. For example, it has been
found that lint having such a transparent film deposited upon it according to this
invention acts very beneficially as a preventive to the formation of pus when
applied to wounds. Plasters, bandages, paraffined paper, boric-acid powder, and
wound-dressing materials generally, other than lint may, of course, be similarly
treated with a like result. By so filming leather or fabrics either simply or alter-
nately with the application of oil or fatty matter, the appearance, durability, and
resistance to damp or mildew is materially enhanced. Corks likewise may advan-
tageously be filmed to render them capable of withstanding the action of acidulous
or other corroding substances. Again 2 per cent. films produced according to this
invention on paper impart to it a surface admirably adapted for receiving impres-,
sions of engravings etc.; and in order to vary the natural india-tint of the film
when used for this purpose, an addition of from 5 to 10 per cent. of asphaltum,
dragon's blood, or other suitable sublimable material may be made in the
composition of the tablets, the quantity of nitrate of potassium being proportion-
ately increased if necessary adequately to effect their proper combustion. By
filming (with 2 per cent. films) finished photographs or prints according to this
invention, fine effects of softness and heightening of shadows can be produced.
The tin amalgam or other silvering of mirrors, moreover, may have non-porous
films formed from a mixture containing 8 per cent. of nitrate of potassium with
the addition of a suitable amount of a sublimable material adapted to render the
films harder, such as 10 per cent. of copal gum or kauri gum, and deposited upon
the silvering in accordance with this invention to protect it from cracking and
becoming "fogged" by contact with a damp wall. The weight of charge for
these various purposes is dependent upon the degree of antiseptic protection or
colour required. Charges of from 10 to 60 grains per square foot of the area of
the filming-chamber may, however, sufficiently indicate the range. It is, moreover,
obvious that the translucent surface produced on glass or the mat or dull surface
produced on metal, by means of a porous opaque film made according to this
invention may not only be plain but be of any desired lettering or design and this
lettering or design may be either clear on an opaque or mat ground or opaque
or mat on a clear or bright ground. To obtain a clear lettering or design
on an

page_text: opaque ground, the lettering or design may be traced or worked direct upon
glass after opaque filming by removing such parts of the film as are necessary
to represent the lettering or design, in effecting which removal a certain portion
of the film is left below the removed parts and the pressure employed, by destroying
its porosity, renders this portion capable of resisting the action of the etching
acid for the requisite period of exposure. Or the lettering or design may be first
drawn or stencilled on, transferred to, or photographically deposited upon, the
glass or metal in suitable "resist" and a porous opaque film be subsequently
deposited as hereinbefore described; after this, the back of the plate is protected
with Brunswick black or other suitable resist and the plate is passed, (care being
taken to avoid cross currents,) evenly into the etching-agent, which in the case
of glass may be the ordinary fuming aqueous hydrofluoric acid of commerce and
from which, after the requisite number of seconds' exposure, it is transferred to
a bath adapted promptly to arrest the action of the acid-in the case of glass
this bath may be a strong solution of commercial carbonate of sodai.e., "washing
soda" and in the case of metal it may be a whiting-bath-.e., a mixture of
whiting and water; the removal of the "resist" by suitable solvents and a sub-
sequent scrubbing in soap and water with a stiff brush complete the process. To
obtain an opaque or mat design or lettering on a clear or bright ground, one or
more non-porous transparent films of the full capacity of the charge-plate according
to the resisting power required, is or are deposited on one surface of the glass or
metal plate and the lettering or design traced or worked through the film or
films so as to expose corresponding portions of the glass or metal (to facilitate
the working a slight porous opaque film may advantageously be deposited over
the transparent film or films to render the scored part more visible); or a porous
opaque film is first deposited upon the surface of the glass or metal and allowed
to mature, then a fern or other natural object, or a stencil having the ground-
portion removed, is placed evenly upon the porous opaque film and a transparent
non-porous film is produced over the whole, after which the fern or other object
or. stencil is removed; the process is completed in each case by etching to the
required depth in the manner above described.
By carrying the etching to a sufficient depth into glass or metal a matrix may
be formed for obtaining a gelatine or other relief, from which a printing-surface
may be made by any of the ordinary methods; or a photographic negative suitable
for the production of a printing-surface may be obtained by photographing white
etching produced by means of a transparent film upon ruby or orange flashed glass.
To determine with accuracy the requisite number of seconds of exposure to the
action of the etching acid for a porous opaque film, or of minutes for a non-porous
or a reticulated transparent film on glass, a trial-strip of the same glass and
filming is graduated and subjected to the acid for successively increased exposures
corresponding to the graduations, and the results obtained determine the proper
exposures.
The fitness of any given sample of glass, more particularly for use with porous
opaque films, is determined in like manner. It is important to take care that
the glass employed be suitable; kinds yielding double fluorides are unsuitable;
and the glass should be so cleaned and polished that the breath will condense
upon it in an even film free from all irregular markings. It is also important
-that the porous opaque-film should be properly matured, the measure of the
opacity that it yields to the glass being proportional to its maturity; and that
the action of the acid (which in this case must be strong-e.g., the fuming aqueous
hydrofluoric acid of commerce) upon the porous opaque film be promptly arrested,
after due exposure, by plunging the article into a solution of the carbonate of
soda of commerce.e., washing-soda.
It appears advisable for etching-purposes, to employ the non-porous transparent
film within a few hours after its formation, as within a day or two an opacity
begins to appear. On the other hand, the longer the porous opaque film is kept
the better it becomes.

page_text: For etching metal articles after filming them according to this invention, the
etching fluid employed must, of course, be such as to form a soluble compound
with the metal to be treated, and the liquid used to neutralize the etching agent
must be alkaline or acid in accordance therewith.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed I declare that what I
claim is-
1. For the purpose of producing on glass and other materials or articles, coatings
or films of pyrobetulin or pyrobetulin anhydride, compressing together into blocks
or tablets a mixture consisting of finely ground epidermis or outer rind of birch-
bark and an oxygen-carrier such as nitrate of potassium, and burning the blocks
or tablets in a chamber without flame, the blocks or tablets being arranged in
such a manner that they will when ignited either all have an abundant supply
of air or have a small supply of air, according to the kind of product required,
substantially as described.
2. Producing on articles or materials porous opaque films or non-porous or
reticulated transparent films of or containing pyrobetulin or pyrobetulin anhydride
respectively, substantially as hereinbefore described.
3. Apparatus for producing films on articles comprising a closed chamber having
a cover below and centrally to which is arranged a lamp having a chimney,
and having agitating-apparatus furnished with a cylinder, over the lamp for mixing
the fumes together and with the air of the chamber, the arrangement being such
that the fumes from the lamp will rise up within the cylinder and be deflected
downward between the cylinder and the cover, substantially as herein before
described.
4. In apparatus of the kind referred to, a lamp having a cup perforated in its
lower part and provided above its perforations with a floor-plate and a charge-
plate adapted to hold tablets or small blocks of combustible material either apart
and with an abundant supply of air (Figs. 6 and 8) or close together (Figs. 9
and 10), substantially as herein before described.
5. Apparatus for producing films on articles or materials constructed and
arranged substantially as hereinbefore described and shown in Figs. 1 to 8 inclusive
of the accompanying drawings or as modified according to Figs. 9 and 10 thereof.
Dated this 14th day of February 1898
For the Applicant,
W. LLOYD WISE,
46, Lincoln's Inn Fields, London, W.C.,
Chartered Patent Agent.