Archive for the ‘Clays’ Category


17 PMpWed, 28 Apr 2010 19:30:50 +020030Wednesday 2010

Hello to everyone!

Pietro sent me the file with all the recipes you guys used for the soda firing workshop. You can download it and add more recipes, if you want, and see that everything is written correctly. And, please, post this again if you change something! Thanks



Tests, which call for further explorations and experiments

17 PMpWed, 28 Apr 2010 12:38:38 +020038Wednesday 2010

As we have tested many local materials (clays from all over Europe, brought by the participants) we have learned quit a lot about their look in soda firing. We have also seen the wide possibilities, which surely will lead to more experiments.

In my case the experiment will start immediately after building new wood kiln with the chamber for soda firing as the one I have right now is not suitable for soda firing (it is soft brick and need to last another year for our woodfiring).

For now I can share the results from the firing in La Meridiana:

All pots fired in gas kiln, reduction, approximatly to Seger cone 8-10 (1250 – 1300°C), soda introduced after cone 8 partly down.


Clay tested: Polar VV – from  H+K Lubná ( – first picture show the clay not fired and second show this clay fired in woodkiln to 1250°C

Slip tested:

Nepheline Syenite 33
Kaolin Sedlec Ia 33
Silica 33
Fe2O3 1,5
TiO2 13,5



Clay tested: cca 1/2 M11 salt-Wittgert / 1/2 Stoneware-St.Amand

Slip tested:

Gail Nichols Rutile

Caolin                  80

Nepheline Syenite         10

Quarz                               10

Bentonite                          3

Rutile                                 3

Borax                                 3

Note: Slip applied in different thicknes



Clay tested: Polar VV – from  H+K Lubná (

Slip tested:

Kaolin 40
Witgert 40
Borax 5
Ultrox 10

Note: Slip applied quit thick



Clay tested: 1/2 shamot clay for shiedel chimney (the inside tube)  / 1/2 Polar VV – from  H+K Lubná (

Slip tested:

Kaolin 40
Witgert 40
Borax 5
Ultrox 10

Results: Slip applied quit thick

CLAY notes by P.E. Maddalena

17 PMpSat, 17 Apr 2010 21:34:17 +020034Saturday 2010

Most clays may be vapour glazed with a various degree of success. As a basic principle the more silica in the clay the smoother the glaze will be, but with less silica and more alumina, the more distinctive “orange peel” texture will result.

The colour of the clay after firing is governed by the nature of the firing and by the iron content. From pale gold (1%) to pale tan (1.5%) to medium brown (1.8 -2 %). More than 3% of iron will cause the glaze to be progressively duller.

Not all clays are well fluxed by the soda because many contain insufficient free silica, e.g. china clays, ball clays and stoneware clays with alumina/silica molecular ratios between 1 to 2 and 1 to 4. The best clays for vapour glazing are those fireclays which are high in silica. These can have alumina/silica ratio of 1 to 5 or higher.

Extra silica is sometimes added to clay as quartz sand but silica in isolation does not make a glaze, some alumina is necessary. Small amounts of calcia and magnesia help to stabilise the glaze.

When the kiln is soaking at high temperatures, various forms of quartz develop in the body of the pots: some of this quartz is very unstable, the most unstable form being cristobalite.
As already mentioned, the more silica in the body, the better it will salt. However, the more silica in the body the higher the danger of cristobalite to develop in quantity and pots to shatter unless due care is taken.

The most stable of quartz forms is mullite which forms if the kiln is cooled very rapidly. Therefore fast cooling down to around 1000 °C has developed thus stabilising the pernicious effects of excessive silica.