Terracotta: The Intrinsic Qualities
- From the Latin, Cooked Earth.
- Formed by the disintegration & de-composed Igneous and Metamorphic rock.
- Either Residual (unmoved) or Sedimentary (transported).
- A colloidal lusterless substance that absorbers water and is then plastic and flexible.
- Durable when heated about 1100'F.
- Rich in Feldspar in the form of crystaline grains.
- Contains Iron oxide and Sulfur as fluxes and colorants.
- Terracotta is vitreous (5 and 15%) usually fired to cone 04 (1943'F).
- Sintering takes place when the mineral particles become fused during heating.
- Withstands deformation while retaining shape after pressure is released.
- Mainly Hydrated Aluminium Silicate, called Kaolinite.
- Has a regular structure of flat, shinglelike shapes.
- The particles tend to slide together and therefore give support to one another.
- Particle size 0.7 microns in diameter and 0.05 microns thick.
- The water content lubricates the plates allowing them glide over each other, this actions gives the clay plasticity.
- Clay remains plastic as long as it is sufficiently wet.
- As the water evaporates the particles are drawn closer together, the clay becoming harder by stages.
- Different techniques are possible at different stages; from liquid (slip pouring) to dry (ram pressed) sgraffito.
- Has porosity and is brittle after firing.
- Breakage releases tension in the terracotta, causing the material to expand, making it impossible to replace the particles in their original position.
- Capable of being pressed into shapes and molds, it remains plastic enough to allow for changes and/or finishing.
- It has no intrinsic shape discernible by the naked eye.
- Takes any form within its structural capacity.
- Inert and permeant to a reasonable degree.