Gail Dapogny on tue 15 oct 02
Ron, I just read your comment to Jose. I had read his post, and realized
that I'd experienced the same problems in a couple of firings, although my
unglazed ( crawled) dots were much smaller than his, evidently-- only about
3/8 of an inch in diameter. Mine happened with at least 2 glazes and I
believe usually in the overlap, so I assumed that thickness might have been
a factor. Maybe not? Also it happened in only two firings. Anyway I was
interested in your list of ideas.
Was wondering if sanding the porcelain lightly with fine sand paper would
help the situation. Also Do you have a favorite ball clay that you sub for
kaolin?
Another porcelain-related problem that I have had occur much more and which
aggravates the daylights out of me is for fine surface cracks to occur on
the surface after the glaze firing (cone 10--gas reduction)-- porcelain
only. This has occurred frequently with several glazes (but not all), and
not only glossy ones.
Have you any ideas about this? I suppose--sigh-- that it is a fit problem
between clay and glaze.....? Are these two problems related at all? The
first problem (crawled areas) happened only twice, but the fine cracks are
an ongoing problem and haven't shown up prior to firing (although I didn't
look with magnifying glass and will do so). Interested in anything and
everything you have to say, and thanks as always.
----Gail
>Hi Jose,
>
>This crawling - and it does seem to happen more on porcelain than stoneware.
>
>Glaze gas to get a certain amount of "grasp" on a pot to not crack and peel
>back during sintering (beginning of the firing) The fine nature of clay
>particles in porcelain are not as conducive to holding glaze as with
>stoneware.
>
>There are some rules to follow - first - use a magnifier to examine the
>glaze after it has dried (completely) on the ware. Many glaze crack at
>least a bit at this stage but not enough to show up after firing - but they
>are a sign that trouble may be around the corner.
>
>Don't try to smooth the glaze - there are much better and safer ways to
>cure the problem.
>
>Don't fire such glazes till they are dry - loading and firing wet glazes is
>one way to get crawling in some glazes.
>Flocculated glazes need more water - shrink more - and therefore crack up
>more during drying and early stages of firing - sometime pieces of glaze
>fall off the ware onto the shelves.
>
>Replacing some or all of the GB with Frit is a good solution.
>Replacing kaolin with ball clay makes the surface tougher and less likely
>to crack.
>Defloccing the glaze will work but adding solubles to glazes should only be
>a last resort.
>If you would like to send me the worst offender I will send it back fixed
>as an example of how to do it Jose - been there - done that - many times.
>
>RR
>
>
>>I have been using a mid range porcelain (cone 4 to 6). I bisque to 06 and
>>fired with a Gerstley borate based glaze to cone 6. I have used these
>>glazes successfully on stoneware, but on the porcelain the glaze is fine
>>and seems to be a good fit, but it leaves some bare spots. I apply the
>>glaze by dipping and the glaze seems to adhere well, but then it moves
>>during firing leaving these bare spots. So the glaze is fine except for
>>these bare spots. The bare spots are not very big (say a quarter to half
>>dollar) and of random shapes with smooth edges.
Gail Dapogny
1154 Olden Road
Ann Arbor, MI 48103-3005
(734) 665-9816
gdapogny@umich.edu
http://www.silverhawk.com/ex99/dapogny (single historical photo - no longer
registered with Silverhawk)
Ron Roy on mon 21 oct 02
Hi Gail,
Sanding might help - you will see if it does - I did have to deal with
crawling once - with my Tenmoku (on porcelain) - subbed in Bell dark and
have had none since. I also have to deal with it when fixing Tuckers
customers glazes and subbing ball clay usually works - it's simply tougher
than Koloin - has a higher MOR (modulus of rupture) and is harder to break
in the raw state. If you have OM#4 it will work I think - send me the
glazes if you don't know how to do it with calculation.
The cracks after the firing - are the edges of the glaze sharp or melted -
I have to know this - what forms does it happen on? You may have to break a
piece to see what is happening.
If it's a fit probelm the cracks are usually all the way to the center or
complete.
Are the cracks easy to see - open or closed?
RR
>Was wondering if sanding the porcelain lightly with fine sand paper would
>help the situation. Also Do you have a favorite ball clay that you sub for
>kaolin?
>
>Another porcelain-related problem that I have had occur much more and which
>aggravates the daylights out of me is for fine surface cracks to occur on
>the surface after the glaze firing (cone 10--gas reduction)-- porcelain
>only. This has occurred frequently with several glazes (but not all), and
>not only glossy ones.
>
>Have you any ideas about this? I suppose--sigh-- that it is a fit problem
>between clay and glaze.....? Are these two problems related at all? The
>first problem (crawled areas) happened only twice, but the fine cracks are
>an ongoing problem and haven't shown up prior to firing (although I didn't
>look with magnifying glass and will do so). Interested in anything and
>everything you have to say, and thanks as always.
>----Gail
>
>
>>Hi Jose,
>>
>>This crawling - and it does seem to happen more on porcelain than stoneware.
>>
>>Glaze gas to get a certain amount of "grasp" on a pot to not crack and peel
>>back during sintering (beginning of the firing) The fine nature of clay
>>particles in porcelain are not as conducive to holding glaze as with
>>stoneware.
>>
>>There are some rules to follow - first - use a magnifier to examine the
>>glaze after it has dried (completely) on the ware. Many glaze crack at
>>least a bit at this stage but not enough to show up after firing - but they
>>are a sign that trouble may be around the corner.
>>
>>Don't try to smooth the glaze - there are much better and safer ways to
>>cure the problem.
>>
>>Don't fire such glazes till they are dry - loading and firing wet glazes is
>>one way to get crawling in some glazes.
>
>>Flocculated glazes need more water - shrink more - and therefore crack up
>>more during drying and early stages of firing - sometime pieces of glaze
>>fall off the ware onto the shelves.
>>
>>Replacing some or all of the GB with Frit is a good solution.
>>Replacing kaolin with ball clay makes the surface tougher and less likely
>>to crack.
>>Defloccing the glaze will work but adding solubles to glazes should only be
>>a last resort.
>>If you would like to send me the worst offender I will send it back fixed
>>as an example of how to do it Jose - been there - done that - many times.
>>
>>RR
>>
>>
>>>I have been using a mid range porcelain (cone 4 to 6). I bisque to 06 and
>>>fired with a Gerstley borate based glaze to cone 6. I have used these
>>>glazes successfully on stoneware, but on the porcelain the glaze is fine
>>>and seems to be a good fit, but it leaves some bare spots. I apply the
>>>glaze by dipping and the glaze seems to adhere well, but then it moves
>>>during firing leaving these bare spots. So the glaze is fine except for
>>>these bare spots. The bare spots are not very big (say a quarter to half
>>>dollar) and of random shapes with smooth edges.
>
>
>Gail Dapogny
>1154 Olden Road
>Ann Arbor, MI 48103-3005
>(734) 665-9816
>gdapogny@umich.edu
>http://www.silverhawk.com/ex99/dapogny (single historical photo - no longer
>registered with Silverhawk)
>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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