Improvements in Enamelling Processes for Electric Heating Apparatus
Year: 1896

page_text: Date of Application, 20th Nov., 1896
Complete Specification Left, 20th Aug., 1897-Accepted, 30th Oct., 1897,
PROVISIONAL SPECIFICATION.
Improvements in Enamelling Processes for Electric Heating
Apparatus.
We ROOKES EVELYN BELL CROMPTON, Engineer and EDMUND JOHN Fox, Engineer,
both of Mansion House Buildings, London, E.C. do hereby declare the nature of
this invention to be as follows:-
Our
In the manufacture of electric heating, cooking and other apparatus in which the
resistance wires are embedded in and attached to the surfaces of plates to be heated
electrically and some of which are described by one of us (the said R. E. B.
Crompton) in Specification of Patent No. 17091 of 1892 as well as by one Charles
Edwin Carpenter in Specification of Patent No. 6072 of 1891 we find it sometimes
necessary to provide special means of increasing the insulation resistance between the
wire and the plate or support to which it is attached by the enamel.
present invention relates to improvements in manufacture by which this resistance
can he increased and the same improvements enable us to reduce the cost of
inanufacture by affording greater certainty in ensuring that during the process of
enamelling the wire does not sink through the innermost coat of enamel and so get
wholly or partly in contact with the metallic support. According to our present,
improvements we interpose between the wire and its support a granulated groundwork
of a very infusible insulating material and by the use of this granulated groundwork
we find that the likelihood of the wire being able to sink or even force its way
through to the metal supporting surface is very greatly reduced. The method
which we prefer for applying this granulated groundwork is as follows.
The metallic surface to which the wire is to be attached by enamel is first coated
by a thin coat of enamel which can be powdered on as a dry enamel or attached by
a wet process either processes being well known in the arts; after this first coat of
enamel has been fused but while still in the molten stage a layer of granulated
material is powdered on. This granulated material may consist of biscuit ware or
porcelain which has been already fired and then ground and sifted through sieves
such as a No. 40 sieve. Sufficient of it is sifted over the molton surface of the enamel
so as to embed itself firmly in it. A further supply of similar ground and sieved
material of a finer grain corresponding say to a No. 60 sieve is also sifted over the
enainel so that it will lie upon the surface of the molten layer of enamel and partly
fill up the interstices between the first layer of granulated inaterial. Then we fuse
over this granulated material a thin layer of enamel, and then so regulate the heat
of our muffle that the two layers of enamel are fused together to a sufficient extent.
to hold the granulated porcelain sandwiched between them. The wire is then
transferred by any of the transfer meaus referred to in former specifications takeu
out by one of us on to the upper surface of this ground work, or in some cases we
find that we can transfer wire to the upper surface by a simple process of annealing
or softening the wire after it has been corrugated and then arranged into any
required pattern. If we employ the wet process of enamelling we can sift the layer
of granulated material into it after the article has been dipped and before firing in

page_text: aome cases by interposing a drying process for a sufficient time. After the
granulated material has been sifted over the surface, we can dip a 2nd time and
thus avoid complications in the firing processes in the muffle. In every case, our
object is to secure a layer of a definite thickness of granulated material
corresponding to the size of the granules of infusible granulated material, which is
sifted in to the enamel.
It will be gathered from the above processes that what we require in the
granulated material is that it should be such a material that it will not fuse or even
if it softens should soften very little at the time at which the enamels are required
to be fused and that further it should be of such a nature that the enamel will
readily fuse to it. For this purpose we find properly ground and sifted crystals of
quartz of certain infusible classes of ground porcelain, biscuit ware, and similar
substances satisfy these conditions in a sufficiently satisfactory manner. We find
that objects enamelled with this granulated groundwork interposed between the
metallic support and the wire do in practice maintain their insulation resistance in
a better and more efficient manner than those enamelled with homogeneous enamel
even if applied in one or more layers and this is especially the case at high
temperatures as we find that a very large proportion of the euamels used in the arts
lose to a certain extent their insulating quality at these high temperatures so that
only a very thin layer of enamel is interposed between the wire and the support;
at such times when this thin layer is greatly heated there is considerable risk of a
breakdown of the insulation at this point but the granulated system of groundwork
berein described obviates this difficulty not only by greatly increasing the specified
resistance of the compound material which we describe but also by the regularity of
the thickness of the layer of insulating material interposed between the two.
In cases where we require to use currents of high EMF it is of great importance
to maintain a very high standard of electrical resistance and in these cases we
sometimes interpose between two layers of enamel a thin continuous sheet of
porcelain or material of a similar nature which will stand the beat of enamelling
without cracking or distortion and we find that by enamelling the surface to be
heated as well as the adjacent face of the porcelain sheet we can attach the latter
firinly to the surface to be heated and again we can attach our heating wire firmly
to the outer surface of the porcelain sheet and the whole can then be fused together
in such a manner that we obtain very perfect insulation of the wire from the
metallic body or support and without diminishing to any serious extent the thermal
conductivity of the insulating material. In this latter case we can whenever it is
convenient to do so attach our wires by sewing then or by lacing them through
holes or by other suitable means to the outer surface of the porcelain sheet prior to
its passing through any of the enamelling processes.
Dated the 20th day of November 1806
WM. BROOKES & SON,
55 and 56 Chancery Lane, London, Agents for the Applicants.
COMPLETE SPECIFICATION.
Improvements in Enamelling Processes for Electric Heating
Apparatus.
We ROOKES EVELYN BELL CROMPTON, Engineer and EDMUND JOHN Fox, Engineer,
both of Mansion House Buildings, London E.C. 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 :-
We find that in manufacturing electrical heating, cooking, and other apparatus by
the processes described by one of us in the Specification of Letters Patent No. 17091

page_text: of 1892 a difficulty presents itself during manufacture namely that the resistance
wire sometimes has a tendency to siuk through the enninel when it is in a fused
state during manufacture and thus comes into contact with the metal plate and the
article is spoilt. In addition to the method described in the said specification of
Letters Patent of employing a harder enamel for the first coat than that for any of
the successive coats we sometimes fiud it necessary to interpose a granulated ground
work the granulations being made of a very infusible insulating material which does
not fuse at the temperature at which the enamel is fused so that the granules of this
inaterial do not allow the wire to sink through them and thus come into contact with
the metal plate. One method of applying this granulated ground work is as
follows:-
"
The surface to be enainelled is first coated with a thin coat of enamel which can
be powdered on by the "dry process" or it can be dipped by the "wet
process
after this cont of enamel has been fused and while still in a plastic state a thick layer
of granulated porcelain or other suitable and infusible insulating material is
powdered on. A convenient size for the granules is that they should be such as to
pass through a No. 40 sieve, (that is to say a sieve with 40 equal holes to a square
inch) this granulating material embeds itself in the plastic layer of enamel. The
granulating material may be applied in one or more coats and a second layer of
enamel may be applied by either the dry or the wet process and the whole will fuse
together so that we obtain a continuous layer of infusible granulated porcelain
sandwiched between two layers of enamel and the whole firmly attached to the plate.
The article can then be returned to the mute and the wire transferred to it and
afterwards covered by a fiual layer of enamel as before described by one of us in the
above mentioned patent specification. In cases where it is of great importance to
maintain a very high degree of electrical insulation at exceptionally high
temperatures we sometimes in place of the granulated material interpose a thin sheet
of porcelain which may be either continuous or may be moulded into hexagons,
squares or any other convenient forms which can be laid on side by side and so
closely cover the surface. In other respects the above mentioned process of
enamelling can be followed. In this latter case we sometimes find it convenient to
attach the wire to the outer surface of the porcelain sheet prior to its attachment to
the surface which is to be heated.
Having now particularly described and ascertained the nature of our said inven-
tion, and in what manner the same is to be performed, we declare that what we
claim is :-
1. The herein described method or means, for supporting resistance wires from
coming into contact with the supporting plate in the article to which the resistance
wires are to be attached for heating purposes, by the introduction of a layer of
granulated porcelain or similar or equally infusible and electrically insulating
substance between two layers of enamel.
2. The method of increasing the electrical resistance between the resistance wire
and the metal supporting plate, by interposing a sheet of porcelain or similar
infusible and electrical insulating substance, substantially as herein described.
Dated the 19th day of August. 1897
WM. BROOKES & SON,
55 and 56 Chancery Lane London, Agents for the Applicants.