Snail Scott on mon 29 mar 04
At 09:39 AM 3/29/04 -0500, you wrote:
>....We
>tried making our own bone china slip...the doll
>heads we tested with both collapsed a fair bit.
Bone china is generally not expected to stand
up well when fully vitrified. If your forms are
such that they're prone to 'flop-osis', try firing
a little lower than true vitrification. For dolls'
heads (that was the project, yes?), complete
vitrification shouldn't be needed anyway.
-Snail
Robert Galejs on mon 29 mar 04
We've made some progress on our hunt for a white porcelain slip. We
tried making our own bone china slip from a recipe we found in a book:
30 Kaolin (English Grolleg)
45 Bone ash
22.8 Feldspar (G-200)
2.2 Silica (Flint 325 mesh)
This was mixed with water to a specific gravity just above 1.7 and then
defloculated with Darvan 7 to a pourable consistency. I thought the
book indicated firing to cone 8. We have tried two firings at cones 8
and 6. Both came out very white, but also very glassy and the doll
heads we tested with both collapsed a fair bit. We are looking for a
matte finish that will readily accept china paint.
Are we just firing way too hot?
Any suggestions for a better cone to use?
Maybe we should change some of the proportions listed above?
What should properly fired bone china look like?
Thanks,
Robert Galejs
Robert Galejs on tue 30 mar 04
Snail Scott wrote:
> At 09:39 AM 3/29/04 -0500, you wrote:
>
>>....We
>>tried making our own bone china slip...the doll
>>heads we tested with both collapsed a fair bit.
>
>
>
> Bone china is generally not expected to stand
> up well when fully vitrified. If your forms are
> such that they're prone to 'flop-osis', try firing
> a little lower than true vitrification. For dolls'
> heads (that was the project, yes?), complete
> vitrification shouldn't be needed anyway.
>
Yes, Doll's heads are the project. We tried a cool cone 5 firing which
worked quite a bit better. The head did not collapse, was still quite
white although still a bit on the glassy side. It looks like we're on
the right track. I'll post again when we find something that works.
- Robert
Andrew Sugden on tue 30 mar 04
Hello Robert,
For what it is worth my thoughts are:
From your description of the fired piece being glassy it does sound as if
you are over firing, or looking at it another way the body is overfluxed
for that schedule. The collapsing of the heads may also be related due to
excessive pyroplasticty.
Two approaches can be taken
1. Softer firing, though being less familiar with cones than other
pyrometric devices I would not like to suggest a particular cone value.
2. Reformulate the body. The recipe you describe is not that far from the
classic bone china composition of 25% kaolin, 25% bone ash and 25% china
stone. G-200 feldspar is a stronger flux than C.stone as it is quite a
high purity K spar.
Looking at the listed recipe I do wonder what the benefit of just 2.2%
silica, and certainly measured to that degree of precision. A start of a
trial might be
Grolleg 35
Bone ash 45
G-200 20
How about mixing up small batches of various compositions, forming a few
simple test pieces and firing to your desired schedule? Such as the above
but with 2% increases on the kaolin at the expense of the flux.
Also do you want the dolls heads to eb bone china or will another body
type be OK providing it is white?
Robert Galejs on tue 30 mar 04
Andrew -
Thanks for the information and suggestions...
Andrew Sugden wrote:
> Hello Robert,
>
> For what it is worth my thoughts are:
>
> From your description of the fired piece being glassy it does sound as if
> you are over firing, or looking at it another way the body is overfluxed
> for that schedule. The collapsing of the heads may also be related due to
> excessive pyroplasticty.
So, if we wanted to stick to the same firing schedule, we should try a
new formulation with a smaller amount of flux in it. Right?
>
> Two approaches can be taken
>
> 1. Softer firing, though being less familiar with cones than other
> pyrometric devices I would not like to suggest a particular cone value.
That's what we're working on now. But one concern we have is that we may
end up underfiring at some point, which (according to my wife) can cause
"mildew" or black spotting during the china paint firings.
>
> 2. Reformulate the body. The recipe you describe is not that far from the
> classic bone china composition of 25% kaolin, 25% bone ash and 25% china
> stone. G-200 feldspar is a stronger flux than C.stone as it is quite a
> high purity K spar.
So it looks like we may have inadvertantly made up a lower than normal
firing temp bone china slip.
>
> Looking at the listed recipe I do wonder what the benefit of just 2.2%
> silica, and certainly measured to that degree of precision. A start of a
> trial might be
> Grolleg 35
> Bone ash 45
> G-200 20
That's a good question about the silica. I certainly didn't measure it
to that level of precision, but I shouldn't be far off. What role does
silica play in a porcelain clay?
>
> How about mixing up small batches of various compositions, forming a few
> simple test pieces and firing to your desired schedule? Such as the above
> but with 2% increases on the kaolin at the expense of the flux.
That's a good idea. We don't have much experience at this and didn't
know how sensitive the recipe might be. Experimenting with the
formulation until we get a good result is probably the way to go.
>
> Also do you want the dolls heads to eb bone china or will another body
> type be OK providing it is white?
Bone china is not a requirement, but it seemed to be one of the whiter
porcelain formulations. Other porcelain types would be fine, as long as
they are as white and have a uniform matte finish. There's several
other desirable traits that boil down to durability of the partially
fired heads. ( Dolls head are typically soft-fired prior to carving out
the eye-holes and having the mold lines removed to cut down on dust).
Any suggestions for another body type to try?
Thanks again,
Robert Galejs
John Rodgers on tue 30 mar 04
In my Alaska shop we fired a lot of slip cast porcelain dool heads.
Always fired to a cone 6. Anything above that always created first
warpage then collapse. At cone 8 you are too hot. Try the cone 6 or even
a cone 5 to 5-1/2.
Also, use shelf cones to determine maturity instead of the mini-cones in
a sitter. You will find there is a difference in temperature and time at
which the cone bends. Not a lot, but some, and it will make a difference
in the finished piece. I always set a shelf cone on a post near a peep
hole so I can look at it once in a while.
Regards,
John Rodgers
Chelsea, AL
Robert Galejs wrote:
> We've made some progress on our hunt for a white porcelain slip. We
> tried making our own bone china slip from a recipe we found in a book:
>
> 30 Kaolin (English Grolleg)
> 45 Bone ash
> 22.8 Feldspar (G-200)
> 2.2 Silica (Flint 325 mesh)
>
> This was mixed with water to a specific gravity just above 1.7 and then
> defloculated with Darvan 7 to a pourable consistency. I thought the
> book indicated firing to cone 8. We have tried two firings at cones 8
> and 6. Both came out very white, but also very glassy and the doll
> heads we tested with both collapsed a fair bit. We are looking for a
> matte finish that will readily accept china paint.
>
> Are we just firing way too hot?
> Any suggestions for a better cone to use?
> Maybe we should change some of the proportions listed above?
> What should properly fired bone china look like?
>
> Thanks,
>
> Robert Galejs
>
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Andrew Sugden on wed 31 mar 04
Hello again Robert,
I am glad that my earlier post seemed to be of some interest, and to
respond to your comments:
1) ...So, if we wanted to stick to the same firing schedule, we should try
a new formulation with a smaller amount of flux in it. Right?
Yes =96 but as mention before try a few small batches were different levels
of flux. Small, simple trials help to avoid alchemy.
2) ...But one concern we have is that we may end up underfiring at some
point ...
Again trials will help determine the firing range of your body. Once
you=92ve settled on the formulation shape a few simple pieces fire say two
or three at a range of schedule. The assessment of these will depend on
what facilities you have access to but may include water absorption bulk
density, firing contraction or even simply are the pieces for purpose.
3) ...What role does silica play in a porcelain clay?
Most whiteware bodies can be described as being tri-axial, ie made from
the three basic components of clay, flux and filler. Silica is a filler
and whilst sometimes erroneously considered inert fillers profoundly
affect all physical properties, especially; the packing density, drying
shrinkage and thermal expansion, as well as being significant contributors
to any glass phase formed. However in the formulation you described I
really can not see that 2% silica is doing much.
4) ...Bone china is not a requirement, but it seemed to be one of the
whiter porcelain formulations. Other porcelain types would be fine, as
long as they are as white and have a uniform matte finish. There's several
other desirable traits that boil down to durability of the partially
fired heads. ( Dolls head are typically soft-fired prior to carving out
the eye-holes and having the mold lines removed to cut down on dust).
Any suggestions for another body type to try?
I agree that bone china does have a particularly white appearance, and
despite being a vegetarian I think it may be my favourite body type.
Anyway you want white but not necessarily bone china ... now I am not sure
what raw materials you have ready access to be when selecting for good
whiteness avoid high Fe2O3 and TiO2. Certainly the feldspar and kaolin you
note are pretty good in respect to white firing so if you can get a white
firing silica try something as simple as
Grolleg 50
Silica 25
G-200 25
Now it may be underfired at your desired schedule or it may be over fired.
However it is a start point for your trials. After a few assessments and
modifications to the ratios you should end up with what you want. I have
assumed the dolls heads are not glazed so thermal expansion is not over
critical.
Hope that is of some use and please feel free to email me with any
questions, and hopefully that you have succeeded!
All the best,
Andrew
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