Shrinkage/Machining?

Can you file or sand clay to get an exact size after the first firing?
IOW, will it shrink more during the second firing?

yes you can sand bisque although it may take a lot of time if you need
to take a sizeable ammount off( and there are a lot of dust particles
as you can imagine) so wear a mask and goggles as bits tend to fly off
and go straight for the eyes.as for more shrinkage this should not
happen if the clay was bisqued to maturity.

That depends on how high a temperature you to fired it to initially. If
there is to be a second firing and the succeeding temperature is the
same or less than the first firing, there may be a small increase in
shrinkage due to the increase in heatwork.
If your first firing was low, to enable you to work on it, and you are
going a lot higher second time around i.e. to stoneware, then the
increase will be substantial. Total shrinkage with stoneware clays is
generally in the region of 12 + percent, but this does depend a lot on
the composition of the clay used; some shrink more than others. Porous
Biscuit Fired ware is theoretically able to be sanded or filed.

Steve
Bath
UK


In article , Noname
writes

Can you file or sand clay to get an exact size after the first firing?
IOW, will it shrink more during the second firing?



--
Steve Mills
Bath
UK

I recently read a great idea on one of the poster's site (I will have to
look it up on a different computer). Take a strip of clay that has been
rolled/flattened out the size of a ruler. Take a ruler that has raised
marks on it and push it into the clay. Fire it and then used the initial
ruler to mark the original inch markings on the fired ruler or just use your
original ruler to find the size you want. So if you want the opening to be
4" when finished use your fired ruler and find what measure on it reaches
the 4" of the original ruler. If you are getting around 12% shrinkage, your
fired ruler would show about 4.5+" on the unfired ruler at 4". I imagine
you would want both a bisque fired and a glaze fired ruler. Though I can't
say how much variance there is going to be in firings and shrinkage. This
probably isn't much help for the moment but maybe something to use in the
future....
DKat


"Steve Mills" wrote in message
news
That depends on how high a temperature you to fired it to initially. If
there is to be a second firing and the succeeding temperature is the
same or less than the first firing, there may be a small increase in
shrinkage due to the increase in heatwork.
If your first firing was low, to enable you to work on it, and you are
going a lot higher second time around i.e. to stoneware, then the
increase will be substantial. Total shrinkage with stoneware clays is
generally in the region of 12 + percent, but this does depend a lot on
the composition of the clay used; some shrink more than others. Porous
Biscuit Fired ware is theoretically able to be sanded or filed.

Steve
Bath
UK


In article , Noname
writes

Can you file or sand clay to get an exact size after the first
firing?
IOW, will it shrink more during the second firing?



--
Steve Mills
Bath
UK

"Noname" wrote in message
...

Can you file or sand clay to get an exact size after the first firing?
IOW, will it shrink more during the second firing?


Most people bisque to about 1000C and glaze fire considerably higher than
that. Which means that you will get shrinkage in both firings.
If you want to make things exactly the right size you need to make a slab,
mark ruler marks on it and bisque and glaze fire it, find out the shrinkage
rate of your clay in your kiln and then calculate the size you need to make
to get the size you want.

"annemarie" wrote :

you will get shrinkage in both firings.

Rats!

If you want to make things exactly the right size you need to make a slab,
mark ruler marks on it and bisque and glaze fire it, find out the
shrinkage
rate of your clay in your kiln and then calculate the size you need to
make
to get the size you want.

That seems to be the consensus.

I think I am trying to do something that can't be done. I want to make
my product fit a pre-made base, to do that it needs to be PRECISELY the same
size every time. I have adjusted for the preliminary shrinkage, and was
hoping I could machine the relevant portion to the correct size before the
final firing. But if it is likely to change size again, I will have to
rethink the whole process. Even one tenth of an inch would be way over
tolerance, and so far I am adjusting for over 1/4 inch..

Clever idea, useful

NJ





v

ceramics have been used in engineering applications for hundreads of years.
when i worked in a pyrotechnic company - some 17 years - we only used ceramic
"headers" - the part of the pyrotechnic that has the firing charge & bridge
wire. you get a very strong material, electric insulation, hermetic sealing,
with high temperature & shock resistance.

we held machined tolerances down to +/- .0002 inches.

the manufacture of these parts was slip cast, limited firing, machining, with a
final firing near the same temp as the initial firing. in the pottery world
these parts would be considered bisque fire temperatures.

see ya

steve




Subject: Shrinkage/Machining?
From: "Noname"
Date: 2/7/2004 4:16 PM Pacific Standard Time
Message-id:


"annemarie" wrote :

you will get shrinkage in both firings.

Rats!

If you want to make things exactly the right size you need to make a slab,
mark ruler marks on it and bisque and glaze fire it, find out the
shrinkage
rate of your clay in your kiln and then calculate the size you need to
make
to get the size you want.

That seems to be the consensus.

I think I am trying to do something that can't be done. I want to make
my product fit a pre-made base, to do that it needs to be PRECISELY the same
size every time. I have adjusted for the preliminary shrinkage, and was
hoping I could machine the relevant portion to the correct size before the
final firing. But if it is likely to change size again, I will have to
rethink the whole process. Even one tenth of an inch would be way over
tolerance, and so far I am adjusting for over 1/4 inch..










steve graber