|Monthly Tech-Tip |
Alternate Names: Barnard Clay Sub SG758
Laguna clay states: This is a blend of minerals that closely simulates Barnard (Blackbird) Clay, a widely used clay that contains iron and manganese (within the clay crystal). This material was developed to mimic the effects of Barnard clay when used as a wash or when used in clay body formulations. Though it can be added to glazes as a substitute material, its effects may vary from the original.
Since the material fires very dark and is somewhat plastic it would seem that it could be used as a base for a black burning clay body. However there are concerns with this application since it likely contains raw manganese dioxide.
According to our tests, the maximum fired shrinkage is reached around cone 2 and it expands rapidly and the porosity increases sharply after that (the fired shrinkage of Barnard stays at 15% all the way to cone 6 and the porosity near zero).
This material is much more plastic than Barnard Slip (almost 7% drying shrinkage vs. 4% for Barnard).
These are fired bars of Barnard Slip going from cone 04 (bottom) to cone 6 (top). It is melting at cone 6. Porosity is under 3% and the fired shrinkage above 15% from cone 1 upward. Drying shrinkage is 4% at 25% water (it is very non-plastic). The darkness of the fired color suggests higher MnO than our published chemistry shows.
A 50 gram powdered sample of Laguna Barnard Slip substitute has been washed through 200, 150, 100 and 70 mesh screens. The raw powder is black, however that color washes away during screening revealing the base clay that is being conditioned by additions of black iron oxide or manganese dioxide containing materials (if they are seeking a close match to the original chemistry of Barnard).
Laguna Barnard Slip substitute fired at cone 03 with a Ferro Frit 3195 clear glaze. The very high bubble content is likely because they are adding manganese dioxide to match the MnO in the chemistry of Barnard (it gases alot during firing).
These are fired bars of Laguna SG758 Barnard Slip substitute going from cone 04 (bottom) to cone 6 (top). It is melting at cone 6. The bars are expanded above cone 6 and becoming quite porous. The drying shrinkage is around 7%, it is quite plastic.
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.