There are various approaches: hand building, slip-casting, press moulding,
throwing, modelling, slab building…
Then there are the firing options: electric, gas, wood, raku, smoke…
It’s worth getting professional advice in the beginning of a project. Many suppliers will advise you on how to use the materials you enquire about, but not necessarily advise on the correct route. An hour or so of tuition is worth every minute.
Different clays have different firing temperatues – these are high enough to vitrify the body – otherwise it will be porous. For functional use this is essential, but for sculpture it is not as important, depending on what effect you want.
EXPERIENCE IS INVALUABLE
- Make fragile objects on a sheet of paper on the kiln shelf to avoid breakages when loading the kiln.
- You need to have an idea of the finish you want and work backwards to choose your clay accordingly.
- Clay Suppliers – most suppliers have a technical advisor you can ask for help.
General Building
- Dry clay slowly to avoid cracking – wrap it in plastic or erect a plastic tent over a
large or complicated object to control moisture. - Do not have closed pockets of air and try to make sure walls are no more than
2cm thick anywhere. If you need them thicker – dry them extra slowly. - Dry objects thoroughly before firing.
Types:
Stoneware
Fired at a high temperature to vitrify:1200º-1300ºC
Colour: varies from light grey to dark red, you can get a white
Strong, non-porous body
Can be unglazed or glazed
Earthenware
Fired at a low temperature to vitrify: up to 1150ºC
Colour: varies from creamy-white to red-brown
Not as strong
Can be unglazed or glazed
Porcelain
Porcelain is the toughest, most unforgiving clay – often resulting in cracks and weaknesses – be sure to investigate semi porcelains, grogged and porcelain paper-clays if you are determined to use it.
| Hard-paste: also known as true porcelain | Fired at a high temperature to vitrify: up to 1350ºC. Glassy look. Hard, strong and non-porous. For delicate objects with thin walls and some translucence. Made from china clay (kaolin). |
| Soft-paste | Softer body than hard-paste. Slightly porous. The glaze is fired at a lower temperature than the body and looks like a separate layer. |
| Bone China | A British soft-paste porcelain. Contains a high proportion of bone ash. Very fine and white. |
Plaster for ceramics
- Plaster is used to make moulds for casting clay: you can use wet clay (slip casting) and pour it into the mould making a shell of clay (a bit like making an Easter egg), or you can use plastic clay (out of the bag) to press it into plaster moulds.
- The good thing about plaster is it is absorbent so it sucks moisture out of the clay quite quickly and you can remove it from the mould.
It is worth playing around with this process, but you can also go to professional mould makers when you get a bit more ambitious. - For beginners, instead of buying the best plasters such as Keramicast or Crystacal, the same result is achieved by using Fine Casting Plaster.
- Keep your plaster away from your clay – bits of plaster in clay will cause problems when firing!
Glazes
Glazes fire at different temperatures so you need to pick your glaze for your clay. Learning about glazes could last more than a lifetime – it is all about chemical reactions: between the raw materials within the glazes themselves, between the glazes and the clay you use, and what happens to the glazes at different temperatures.
It is best to take an experimental approach to glazing until you have been doing this for many years, or unless you go to a professional. It is very unlikely the glaze is going to come out how you have imagined it in your head, especially at high temperatures. At lower temperatures (earthenware), outcomes of glazes are pretty reliable, and commercial glazes can be bought to achieve specific colours and effects.
Using pre-mixed glazes – found in most decent ceramics suppliers’ catalogues – could be a good alternative to mixing up found or adapted glaze recipes.
Try testing glazes before applying them to the finished pieces – pop a glaze test in every kiln load.
Technical literature: Ceramics Handbooks Series (A&C Black)
The Potter’s Dictionary of Materials and Techniques by Frank and Janet Hamer (A&C Black)