Michael Banks on thu 27 dec 01
Vivek,
There are several possible causes of this fault. But (presuming that your
glaze recipe hasn't changed), it is probable that the composition of your
black fireclay has varied. You need to do apparent porosity tests on your
fired clay body. If the glaze used is a tried and true one, you may be:
1. Underfiring, allowing post-fire expansion of the overly pororous body, or
2. Overfiring, causing a higher than desirable vitrification of free silica
in the body
For condition No.1 (determined by finding a fired porosity of more than
about 3%): Fire higher. If it is easier to add extra ingredients to the
body, you may wish to add more flux to your clay body (e.g: feldspar).
For condition No. 2 (measured apparent body porosity approaching close to
0%), Fire lower, or add silica sand to the clay (to increase the expansion
rate).
The most likely cause is that the raw black fireclay has become more
refractory (determine this by water absorption tests) or that ware is
emerging from the kiln in an underfired condition (also determined by
adsorption testing). The latter condition may be due to stacking patterns
which obstruct flame travel, or insufficient soak period.
Cheers,
Michael Banks,
Nelson,
NZ
----- Original Message -----------------
Vivek patel wrote:
> Actually I am into manufacturing saltglazed sewer
> pipes and decorative saltglazed tiles in India. I fire
> these in huge downdraft kilns of 24 to 30 feet in
> diameter. These kilns are fired with coal. Lately I
> have been experiencing the problem of crazing on my
> tiles and the tiles are fired as whole boxes and then
> after firing they are split into tiles.
Michael Banks on sat 29 dec 01
Quite so Ivor,
I put it down to imbibing too much Christmas cheer. Vivek's salt would not
likely change composition :)
As you say, saltglazes are often crazed but Vivek's problem is much more
unsightly (I received a jpeg image of the tiles from Vivek). The "crazing"
actually appears to be open cracking of the clay body in a surficial skin of
fluxed clay. On two of the tiles, the glassy skin has flaked off.
The underlying clay body would appear to be too weak and porous to resist
the tension of the fluxed surface layer.
So I stick with my initial conclusion: "The most likely cause is that the
raw black fireclay has become more
refractory (determine this by water absorption tests) or that ware is
emerging from the kiln in an underfired condition (also determined by
adsorption testing).
Michael
in NZ
----- Original Message ---
Ivor Lewis wrote:
Dear Michael Banks <>
In response to Vivek you say <fault. But (presuming that your glaze recipe hasn't changed), >>
What glaze recipe are you talking about? This Vivek Patel said he was were
salt glazing so it is the reaction of the sodium chloride with the clay
which makes the glaze. Nothing is put on the clay. There is no glaze recipe
to change!
As You may know, Glazes produced by "Vapor Glazing" with Salt always tend to
craze. Any that do not are unusual.
Best regards,
Ivor.
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