Terra sigillata is a very smooth, lustrous coating of clay that resembles a glaze and is virtually waterproof.
The name means "sealed earth" and has been used to refer to the Classical Greek Attic black figure and red figure painted pottery.
These days, the name terra sigillata is used to refer to an especially fine coating of clay applied to a pottery or ceramic piece.
For centuries the secret of making terra sigillata was lost and only in the middle of this century was the true nature of this material, the technique of its creation and use rediscovered.
The silkiness and shine of terra sigilatta is due to the plate like shape of the clay particles and the use of only the smallest particles.
Polishing this surface with your hand or a soft cloth lines up all the clay 'plates' and gives the surface its shine.
Most terra sigillatas are made by a process of levigation in water which allows the larger particles to settle to the bottom, leaving the very finest, sub micron sized particles in suspension.
These very fine particles are siphoned off and become the terra-sigillata.
There are many ways to make terra-sigillata.
This is a simple method that works for me.
Add dry or moist clay to a lot of water.
The proportions by weight are usually 1 part clay to 2 parts water.
This can be as high as 1 part clay to 4 parts water for a REALLY plastic clay, like a ball clay.
Mix the clay and water very thoroughly to break down any lumps.
Let the mix sit for a day and mix again.
I do this over several days mixing for about 15 minutes a day.
Mix thoroughly one more time and pour the mix into the largest, tallest, transparent container you can find.
Add about a 1/4 oz. of liquid sodium silicate per 4 1/4 cups and mix thoroughly in the containers
Leave this undisturbed for about 24 hours and it should settle into two or three visible layers.
The layer you want should look like VERY thin milk.
Siphon off the VERY thin, milky layer into another container.
Congratulations, you now have terra-sigillata!
Clays you can use.
You want to start with the most "naturally" plastic clays that you can.
The amount of terra sigillata that you get in the end will depend on the amount of very fine particles in the clay.
Generally the more naturally plastic a clay is, the more terra-sigillata it will yield.
I use the term "naturally" plastic because many clay bodies are only workable because they have had plastisizers, like bentonite, added.
These clays probably won't yield enough fine particles to make your work worth the effort.
In general, porcelains will yield less terra-sigillata than "natural" earthenware clays.
For White or "Colorable" Terra-sigillata, use a very plastic, white burning ball clay. Avoid low fire, white clays.
Many of these contain "plastisizers".
White terra-sigillatas can be colored using Mason stains or coloring oxides like cobalt.
The stains or oxides should be as finely ground as possible otherwise the relatively large size of the oxide particles can interrupt the shine of the sub-micron size terra-sigillata clay particles.
"Natural", low fire earthenware clays like Red Art from Ceder Heights make great TS. Most earthenware clays, through the natural process of their making are very plastic, have LOTS of extremely small particles and yield a lot of terra-sigillata.
They also lend their own natural "earth" colors colors; reds, browns, oranges, yellows, etc.
Color depends on the final burning color of the clay.
Some people treat the surface of the clay before applying the terra-sigillata by either lightly sanding the surface with a very fine sandpaper or burnishing it with the back of a spoon or a smooth stone.
I never did any of this.
Most people I know, brush or spray the terra-sigillata onto bone dry clay, applying several thin coats, letting it dry again between coats.
If you're lucky, you can apply the terra-sigillata to bisque.
This allows the additional option of dipping and will eliminate losses due to handling fragile bone dry clay.
It will take some testing to see if this is possible.
If the terra-sigillata flakes off after firing, go back to applying to bone dry.
How successfully the terra-sigillata hides the body underneath is important to many people.
However, cover isn't everything.
Sometimes it's nice to have some of the body color coming through.
Post Application Treatment:
If it's a good terra-sigillata, just the act of brushing it on should raise a sheen.
I usually leave it at this.
However some people want more of a shine and there are probably as many ways of doing this as there are artists working in terra-sigillata.
I find that lightly rubbing the dry surface of the terra-sigillata with my bare hand will raise a mild shine.
A "Baggies" type plastic bag over my hand will raise quite a bit more shine.
This is about the limit for my tastes.
If I do want to raise a really shiny surface, I lightly brush my finger tips over a block of lard then rub my finger tips against my palm to distribute the lard some more.
Then I start by lightly brushing my finger tips over the surface of the terra-sigillata, gradually making more and more contact with the surface.
You'll find that this can raise quite a shine.
For other methods of burnishing the terra-sigillata look to the work of Vince Pitelka, Michael Wisner, etc.
Terra-sigillata can be fired to what ever temperature you like
As long as it's above the sintering temperature of the clay used to make the terra-sigillata.
Sintering of the clay is required to join the coating to itself and to the body it's covering.
In general, the lower the temperature, the more shine will remain.
The higher you go, the more shine, you'll lose.
However, some people fire their terra-sigillatas quite high.
The ball clay that I use for my white terra-sigillata is a high fire ball clay and still retains a pretty good shine up to about 1200˚ C / 2185˚ F / ^5.
Also, I think that the higher you go, the better the terra-sigillata bonds with the clay body underneath.
Of course, the clay you use to make the terra-sigillata, the clay body underneath, the thickness and method of the application, the firing temperature, etc, as well as your own ideas of "success" will determine if the terra-sigillata is "successful".
Your mileage will vary.
My recommendation is to test making terra-sigillata out of several different clays.
If you're working with students, this can be part of the exploration.
Test each one to see:
How much terra-sigillata does the clay yield?
How it can be applied.
Does it prefer bone dry or can it be applied to bisque?
Does it prefer to be brushed on, sprayed or dipped?
How many coats does it take to hide the body underneath?
Does it like a thick application of many coats or only a few coats?
Can it be burnished easily?
Do you like the burnished surface that it gives?
Does it stay on after firing or does it flake off?
How thick a coat will stay on without flaking off?
How high can the terra-sigillata go without losing it's shine
Copyright 2004 Cindi Anderson, BigCeramicStore.com
|Facts do not cease to be facts because they are ignored.