search  current discussion  categories  glazes - chemistry 

: slumping clay... try "deformation eutectic"

updated thu 21 feb 02


iandol on wed 20 feb 02

Dear Melinda,

I can understand the logic behind your reasoning for your recipe and you =
seem to have chosen a good way to reduce the maturity of a standard =
^10-^12 50:25:25 porcelain down to ^6 in adding several auxiliary =

I have one concern and that is with the purity of your kaolin and ball =
clay. Most mds sheets will give the residuals and there may be up to one =
percent KNaO in a kaolin and over two percent in a ball clay. This may =
be as residual unweathered felspar or some sort of mica, more usually =
the latter. But it can be a significant proportion of the mass of the =
material. This may be the factor you should investigate.

In addition, the marginal amount of Calcium carbonate you are including =
may enter into a chemical reaction with silica to form Wollastonite =
before you get to the melting point of your major flux. This may be =
forming what is known as a "Deformation Eutectic" with your Felspar and =
Talc. The thermal transformation chemistry of the system =
CaO-MgO-Al2O3-SiO2 is complex. It becomes even more so when KNaO is =
added, via felspar and potential mica. Then there is potential for a =
liquid to form below 1200 deg Celsius, which would account for your =
severe slumping problem.

It is interesting to note that though the concept of a "Deformation =
Eutectic" was introduced, I believe by H. Seger in the late 1800s, it's =
value has been ignored by modern writers and it seems not to be =
mentioned in books about glazing or for ceramic artists after the =
publication by S. Singer and F. Singer of Industrial Ceramics.

I will watch for other answers to your problem with interest. They might =
give a good idea how deep the curriculum is, related to Glaze Chemistry, =
at undergraduate and graduate levels.

Best regards,

Ivor Lewis. Redhill, South Australia.