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terra sigillata-black

updated mon 31 mar 97


Jerry Bennett on sun 30 mar 97

Carissa: Black is an easy color to obtain with Terra Sigillatas. I use
this formula:
For Black: Water 800g. (I make only small batches)
OM4 200
Mag Diox. 10
Cobalt Carb. 20
Copper Ox. 20
Black Copper 10
Now for the hard part. How to defloculate the mixture. If you use the
OLD formula of Calgon--per 800g of water use 10g of Calgon for white
clays and 20g for red clays. In recent months the makers of Calgon have
changed the formula for their product. In several states they have
envirotmental rules for potash in water. The new formula for the product
will sightly defloculate clay but I would not recommend that you use it.
The liquid that is common to use to defloculate slips is soda silicate.
(Na20.sio2). This is best used with a very small amount of soda ash.
By the way, you can also use wood ash, but I would also not recommend
this use. For the black formula I would use the soda silicate at a rate
of 1.5g and soda ash at 1g. For red clays use 2g and 1g. By the way, if
you over deflocculate the mixture add additional water, mix and let it
settle for an additional day.
To make the sig, measure out the water to be used. Make sure it is hot
water. Mix in the Calgon, or the sodium silicate-soda ash mixture and
allow it to set for a moment. If you use the Calgon the mixture will
turn white, then blue, when you give it some time. Mix the dry materials
and then mix with the water mixture. A small, tall plastic container is
best to use that allows the clay through the column of water. Stir and
mix the dissolved clay and colorants very well. Let stand for a day.
Water will form on the top of the mixture. I use a turkey baster and
siphon the water off. The Terra Sigillata is the second layer in the
mixture. Pour this layer off. All of the thick layer at the botton of
the container is not to be used. The usual amount for the layers is
1/3-1/3-1/3. Put the middle mixture in a clean seperate container and
the mixture will continue to have a water layer form on it the longer you
keep it. Decant the water and over time you will have a terra sigillata
at the desired thickness. I would recommend that you keep the mixture on
the thin side and use many coats of the sig for the color desired. This
formula works great up to cone 5. I fire most of the terra sigs to 06.
By the way I think this formula for white is also very good:
Water 800g
OM4 150
EPK 150
Talc 300
Zircopax 100
If you make this formula, then add mason stains to the prepared mixture,
you can make any color that you want. Mix the stains and vary the
colors. To figure the % of stains to the prepared mixture you can
calculate the amount of clay in a wet mixture by weighing the terra sig
as a wet mixture and then weighing the same amount of water. Deduct the
water total and the difference is the dry amount. Add the percent of
mason stains.

Hope this helps. Let me know how it works out.
Jerry Bennett