Diana Pancioli on sat 4 oct 03
Hi Tony:
I made some of this while in grad school.
It couldn't stand up by itself.
I fired it in bisqued clay molds.
Just made flat curving shapes, like curled tiles,
for hanging up in mass.
They were very thin and certainly translucent.
Fired at Cone 04. I doubt that you could
make any kind of pots with them.
Diana
P.S. Thanks to those who offered advice about insulation mix.
I appreciate your help.
Janet Kaiser on sun 5 oct 03
Gosh, Diana! You brave lady!
To quote again from Ernst Rosenthal (my little bible on pre-war
manufacture & ceramic production as well as a lot of historical
detail):
"Shaping and Drying Bone China: Because it is less plastic the
bone china body has to be handled more carefully than hard
porcelain bodies, and this applies to both the jollying and
casting processes. Otherwise the shaping processes are the same.
After the shaping processes the ware is transferred to the drying
stoves where, owing to the lean nature of the body, the water is
given off readily. The dried articles have very poor mechanical
strength and great care has to be exercised when handling them
during the cleaning, sponging and placing into the saggers for
biscuit firing. Considerable progress has been made, however, by
the industry in this respect during the last few years."
That was in 1948! But as it was one of the very few books on
ceramics available even when I was at college, as soon as I read
something like that, I knew it was not for me! I seemed to
attract disaster without warnings of exercising "extreme care"
:o) Also when only slip / jollying was mentioned, it sort of put
it outside our ken. I presume some of the "considerable progress"
he spoke about has gone on and resulted in these "low-fired
porcelains" that were recently discussed at some length?
Just to put it on record in case anyone is really interested (and
it keeps my fingers nimble typing all this stuff) Ernst goes
on...:
Bisque Firing: The firing of bone china is quite different from
that used in hard porcelain manufacture and resembles more the
firing of earthenware. The sequence of operations is as follows:
1) Unglazed ware is fired at about 1300 =B0C at which temp. it
becomes vitrified and translucent (bisque-fire).
2) Ware is then glazed
3) Ware is glost fired at about 1100 =B0C and at this temperature
(which is about 200 =B0C lower than the "bisque" temperature) the
glaze materials fuse and the ware is covered with a brilliant
glaze.
The firing of bone china, particularly the first bisque fire is a
very difficult operation because:
1) Bone china has a very high shrinkage which may cause
deformation and cracking.
2) Bone ash in combination with stone and clay produces when it
melts a very fluid liquid. This flux gives the body a very short
firing range.
A very even heat distribution in the bisque oven is of vital
importance for two reasons:
1) If the ware is slightly under-fired it is highly porous and
tends to cause crazing.
2) If the ware is slightly over-fired it deforms rapidly.
Old Ernst goes on to say that kiln size is smaller for bisque
firing than glost, but that is commercial kilns... But reason is
eveness of temperature. Then (and this is interesting from your
experience, Diana): Bone china becomes soft at vitrification
temperature and so, in order to prevent deformation, plates,
saucers and other flat ware are embedded in alumina. Alumina has
completely replaced silica as a bedding material in the bone
china industry.
Bone china hollow-ware and cups are not embedded in alumina, but
are placed in those parts of the oven in which relatively low
temperatures are maintained., If necessary, cups and hollow-ware
are prevented from deforming by supports, rings, etc. made of the
same body.
Glazing:
After completion of the bisque fire, the adhering alumina is
cleaned off by brushing or air jets and the ware is then dipped
into the glaze slip. Since bone china is practically non-porous
it does not absorb the glaze in contrast to earthenware. In the
case of bone china the glaze has therefore to adhere to the
surface and must consequently be more sticky and heavier than
earthenware glaze (to demonstrate this point it may be stated
that comparing one pint of bone china glaze slip with one pint of
earthenware glaze slip, the weights are 32 oz and 28 oz
respectively). In the case of bone china the water in which the
glaze is suspended is not absorbed by the body but remains in the
layer of glaze until it is dried off.
The drying of glazed bone china is therefore a longer and more
important operation than the drying of glazed earthenware. It is
best done by utilising the waste heat from the ovens in mangle or
other continuous dryers.
Bone china glaze may have the following composition:
FRIT:
Borax........................21
Cornish stone.......26
Flint............................21
Whiting......................13
Nitre...........................1
Soda ash...................3
China clay..................15
-------------------------------
100
GLAZE:
Frit............................100
China clay.............2
Cornish stone........4
Flint............................2
White lead...............65
-----------------------------
175
This recipe is take from McNamara "Ceramics" Vol III
Pennsylvania State College, 1944
Owing to the white lead content in the glaze materials, this
glaze is a "full lead" glaze and is not permitted in Great
Britain. By introducing the lead in the form of lead bi-silicate
a low-solubility glaze would be obtained which, while having the
properties as the glaze referred to above, would be less
dangerous to the operatives."
+++end+++
Whew!!! Now I've had enough! Anyone who wants to know about the
glaze firing, Bone China Decoration, "A Visit to the All-electric
Factory of Josiah Wedgwood & Son Ltd., Barlaston" and the rest
can ask!
Night all!
Janet Kaiser --
*** IN REPLY TO THE FOLLOWING MAIL:
>I made some of this while in grad school.
>It couldn't stand up by itself.
>I fired it in bisqued clay molds.
>Just made flat curving shapes, like curled tiles,
>for hanging up in mass.
>They were very thin and certainly translucent.
>Fired at Cone 04. I doubt that you could
>make any kind of pots with them.
*** THE MAIL FROM Diana Pancioli ENDS HERE ***
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