Terra Sigillata, also known as Terra Sig, is a slip which is the suspension of deflocculated fine clay materials in water. It originated in Ancient Greece and flourished in Rome. It uses resurfaced in the 1930s.
Terra Sig is a fine material obtained by separation through sedimentation processes of clay in water. The unwanted heavy particles settle to the bottom and the clay material floats.
It is applied only as a very thin layer on ceramics in the greenware state but preferably when the clay is leather hard. The texture of the clay is not noticeably affected by terra sig.
It is not a glaze so it is not waterproof.
Qualities of good terra sig:
1. The type of clay (the most common clay mineral is Kaolinite)
Material Raw clays versus plastic clays may give a higher gloss at low temperatures.
No fiber clay.
2. The ratio of water to the clay
Sedimentation can only be successful if the mixture is not too viscous; avoid the slip becoming too thick.
If there is too much Water the particles settle quickly. The optimum separation of clay minerals and nonplastic particles do take place.
Clay with high plasticity needs significantly more water compared to clay with low plasticity.
Types: Sodium metaphosphate, sodium silicate, sodium carbonate
Polyacrylate produces good results
Types: Dolapix PC67, Darvan 811,Darvan 7, Dispex
Last ditch effort: sodium silicate
!Clays with high amounts of calcium oxide are difficult to deflocculate.
!Too much deflocculant will counteract the separation process
4. Sedimentation time
Time will influence the final quality.
Non-plastic settles fast
“Coarse” materials settle in short periods of time.
The more coarse the clay is the more opaque and less shiny the terra sig will be.
Too long of a settling time will lead to unwanted settling of fine clay minerals
The most non-plastic materials will settle in 2-3 hours.
After 24 hours no more sedimentation is advised.
Pour out the non-settled liquid part.
6. Maturation time
Maturation is the time allowed to stand before being applied.
Long time = less gloss
a few days compared to one week = noticeable difference.
Allows the clay to be split up into smaller particles which reflect light better.
Smoothness and dryness of the piece will influence the end result.
Smooth surface = more shine
Leather hard = shrinkage and water penetrating through the terra sig layer will reduce gloss significantly.
8. Layer of thickness
Must not be applied too thick or too thin.
Thickness increases the chance of shelling
9. Top Temperature
The gloss of terra sig will not disappear as the temperature rises beyond a certain point.
High temperatures = more vitrification gradually changing the character of the gloss
Fire at cone 04 to cone 1 or 2
!Beyond cone 2 it loses gloss and does not work well under a glaze.
The Ceramic Process by Anton Reijnders
Cushing’s Handbook by Val Cushing