Clay is a special kind of dirt! It has been formed over millennia of grinding up and weathering of regular dirt. Sand has interesting particles that are jagged and not uniform and not organised.
Clay has particles that line up and can be organised. Part of the process we must take when working with clay is to recognise this organisation and respect it!
When we roll out the clay we need to tell those plate like particles how to lie. When we throw the clay we also need to work with the microscopic plate layers of the clay. When you wedge clay you also have to keep in mind what is under the microscope! “Compressing” the clay is very important – this is essentially organising those microscopic sized clay particles! Unorganised clay particles tend to crack, warp and do whatever they feel like when they are drying, and firing!
These fine particles are called “colloidal” particles and they hold together whatever else is in the clay. Clay can come in many different types that fire to various temperatures and perform different duties – some are even in spark plugs in your car, and other fly around in space ready to protect space shuttles during reentry. Different clays have different base chemicals, but most of the clays we use have kaolin as its base colloidal particle. Clays can have texturing particles, such as the Chris’s light and dark speckled clay we use. Colourants can be natural like iron ore and manganese dioxide, or added such as “engobes” or body stains. Vitrification point (when the clay becomes glass like) can be at various temperatures depending upon the type of additives in the clay that act as a flux (a material that causes more refractory – harder to melt – materials to melt at a lower temperature.
Clay is thixotropic to a more or less extent – this means that when it is wobbled it becomes more liquid. And when it rests, it becomes more firm. When you are working with clay you need to keep water to a minimum because as you work, the clay gets more and more unstable. Letting your work rest for 20 or so minutes can really help you built your piece.
Using too much water when you are throwing not only makes the clay more difficult to control due to the thixotropic nature, but also washes out the colloidal particles, making your work more like sand – an therefore less able to hold shape!
What kind of clay do we use at Firebird Studios?
First understand our firing temperatures – at Firebird Studios we use a “midrange” or “midfire” stoneware temperature of 1240°C. Stoneware is technically from 1260 – 1280°C. Midfire is a range that uses clay designed to mimic stoneware at a lower temperature – this saves around 30% in firing costs! Hard to image that 20 degrees can make so much difference at over 1200 degrees!
- Walkers #10 Stoneware (midfire to stoneware temps)
A nice white clay that begins vitrification around 1220°C that is 120 mesh (fine particles). Nice to hand build with but it can warp at higher temperatures, nice to throw with, matches the Walker’s #10 casting slip we use, and is the same clay body as the coloured slip/engobes we use! Walker’s #10 raku fires very well too!
We sometimes call in other brands of fine white midfire clay when Walkers #10 is in short supply, they all perform in a similar manner and will all mix together during recycling.
2. Chris’s Midfire Light Speckle
A light grey clay with small iron speckles – can look amazing with some of the glazes we use. Great for hand building, a little hard on the hands when throwing. The coloured slips fir the clay without issue.2. Chris’s Midfire Dark Speckle
A dark red/brown with fine iron speckles. Can look great with some of the glazes we use.
3. Southern Ice Porcelain
This is a special porcelain clay that work with fineness and translucence at lower temperatures – porcelain is technically fired at 1300 degrees! Southern Ice is fine at 1240, just gaining translucence, is at the first stage of vitrification and comes up a beautiful white after the gloss firing. Porcelain can be tricky to work with as it is very fine and very temperamental! However, Southern Ice is much easier than some!
4. Various Keen’s midfire coloured and textured clay.
These are the darker red and brown clays we have available – good for throwing (can be gritty on the hands) and good for hand building.
If requested, or from time to time, we can mix various coloured clays together to make a natural nerikome (layered coloured clays) t make stripes and interesting wares.
5. Walkers #10 casting slip
We can do some slip casting – and we even make our own simple one or two piece moulds – and the engobes (body stains – colouring for the clay) is carried in Walkers #10 casting slip. Casting slip is very thixotropic – it has additives in the liquid mixture to make it more thixotropic – and should always be well stirred before using, or adding water!
What we don’t use is any earthenware clay or specialty raku clays! If you bring clay to class from outside it is very important you know its firing range and we will often ask for a photo of the label! Clay that is designed for a firing temperature below midfire will melt in the kiln!
Handling the clay:
Normally students will use the clay directly out of the bag without wedging. The clay that comes from the manufacturer has already been through a de-airing pugmill. The pugmill minces and mixes the clay and the de-airing part sucks all the air bubbles out. It does a better job of aligning the clay particles than we can manually! Don’t try to fix what ain’t broken!
On the other hand the clay that I recycle is mixed manually by cutting, layering and compressing over and over. Finally spiral wedging. It has air bubbles in it, and sometimes a few lumps not quite blended really well. We will soon have a de-airing pugmill of our own and we will be able to recycle much better (and with less sweat from me!).
When the bag of clay is open for class, unless it is really hot and windy, no need to cover the top of the bag – the little dry bits fall off onto the clay making it harder to use for the next person. Roll, or wedge your clay if if want to wedge it, on the table for clay handling and take it to your own work space for finishing. Keep your clay on a plastic bag, and scrunch up your clay scraps and keep on plastic as well.
All of the table tops have fibre cement sheeting – this is great for stiffening up your clay for slab building, but not s great for keeping your clay good to re-use. If you want to re-use your scraps they will need a little wedging to work out air bubbles and any lumps.
Once you have finished all scraps, including wheel scraps and water, go into the appropriate bucket for recycling. Don’t empty out your little water tubs on the table – they eventually go into recycling as well. NO colour at all is to go into the porcelain or white clay recycling – there is a bucket especially for all mixed colour clay, Clays like Chrises Light and Dark and Keenes all have their own bucket!
No clay goes down the drain at all!
Clay is the life blood of pottery and getting a feel for how it responds to your touch is one of the many joys of creating with clay. It is also where the series of technical article starts as clay is usually the start of our creative journey!