This is the most common question we get asked in the shop at Mystery Creek Ceramics, it seems like a simple question but there really is no simple answer. This blog post is my attempt at laying out all the considerations when you are picking a clay body to work with. Though I've attempted to cover all elements, I may have missed something—so if you notice that I've overlooked anything let me know.
Please take note here that I've written ‘rules’ in here. ‘Mystery Creek Ceramic rules’ are not hard and fast and can absolutely be broken, we just recommend you use them as a guideline if you're a beginner or have no experience in that particular technique or material. When making our own work at Mystery Creek Ceramics we break ‘rules’ but have a wealth of experience over time to ensure we don't damage equipment.
Step 1. Pick a firing temperature
Different clays have different firing temperatures and this can be critical in some situations.
Clays cannot fire higher than they are recommended without testing. For example I can confidently say walkers school terracotta can go to cone 10 because I have fired it in both a cone 10 gas firing and a cone 10 wood firing.
There are absolutely situations where clays will melt and ruin pots or kiln shelves.
Image from Reddit user MyHandsAreSalmon
This is a costly error. To test I suggest making a small tallish (5cm) cylinder and placing it on a biscuit of known clay (small slab of clay that if it melts it will melt onto without destroying kiln hardware). If it doesn't slump or bloat try a small mug on a biscuit and go from there.
When you're making functional ware (cups, bowls and plates) or sculptures that go outside in frosty areas the firing temperature of a clay is very important because of vitrification. Vitrification is when clay turns to glass so it's a measure of the percentage adsorption of water of a clay body once fired (glass adsorbs 0% water). Now lots of figures are thrown around when it comes to vitrification— some people say 0.05% adsorption is vitrification and some say for stoneware bodies it can be up to 5% and that is vitreous. I don't want to wade into that debate right now but what I'm referring to is that the clay body can hold water without glaze. In a future blog post I'll go into my opinions about functional ware and vitrification but for now we'll leave it at this.
If you are making outdoor sculptures the issue comes with frost. When the clay can adsorb water and then this water freezes (expanding in the process) the sculpture can start to break down or not last as long, so in this situation you would want to get as close to the vitrification temperature as possible.
Image of frost damaged sculptures from Ceramic Arts Daily David Scott Smith
Ok so you have a guide of the firing temperature of clay you need, this will narrow down your clay options.
Step 2 and 3. How you are going to make your project and what type of clay do you want to use?
The technique you will be using to build your project has an impact on the clay you'll select. Some clays do much better than others depending on the technique you choose and some clays are easier to work with than others due to additives and particle sizes. Your level of clay experience in the technique will also have an impact here.
The three most common techniques are throwing (on the wheel), pinch and coil, and slab building.
The most common additives to clay are grog and paper.
Paper is actual paper fibres. You can add your own using the cheapest toilet paper you can find. In my experience it's best to add it to the clay when it's in a liquid state. Most commercial paper clays run at about 3% paper but some people use up to 50% paper when making sculptures. Paper fibres are longer than clay fibres and act like a lattice transporting water throughout the clay body. This has advantages like better joining with less cracking and the ability to re-wet dry projects. We use a paper porcelain to make our nerikomi products and at 3% paper I find it very difficult to re-wet already made projects but the better joining allows us to use the nerikomi technique with less cracks.
Grog is the name for an additive to clay that could be crushed fired clay, sand or a granular material designed to leave speckles in the clay. Grog comes in many different forms and sizes and this impacts your clay decision. Grog can improve the workability of the clay for beginners (in that it holds its form well), can decrease firing and or drying shrinkage, can impact thermal expansion and sometimes prevent cracking. Grog also allows your clay to have more variety of particle sizes and in general this makes clay easier to work with but it can decrease plasticity.
Raku T is a heavily grogged clay
Throwing: You can throw with any clay excluding paper clay (note a ‘rule’ here) as the paper fibres tend to adsorb lots of water and make the clay mushy and unable to be crafted into more complex shapes. Grog impacts throwing depending on the particle size in how the clay feels under your hands on the wheel. For example the Keanes speckle clays have grog for speckle but this can barely be felt by your hands on the wheel whereas BRT is a very heavily grogged clay and can make some potter's hands bleed when thrown on the wheel.
Pinch and coil: Using this technique you can use almost any clay. The thing to take into consideration is your experience with this technique. Clays with minimal size distribution of particles (read white clays) crack more easily than clays with larger variation in particle size (read everything but white clays).
Slab: This can be a trickier technique to pick a clay for as it depends on what type of slab building you're doing. We would recommend a clay with paper or a fine grog here for best results (note a ‘rule').
Using these guidelines will also narrow down your clay selections.
Step 4. What colour would you like?
This is my favourite part! Clay comes in a variety of colours and can be coloured itself.
The natural colours vary from dark brown through all possible variations until translucent pure white. The colour does also vary with firing temperature so make sure the test sample or tile you are looking at is at the temperature you plan to fire the clay at and then choose! Always look at fired samples of the clay rather than the actual wet clay as the colour can differ significantly.
There are speckles, no speckles, every shade of brown under the sun, warm buffs, cool whites and greys. Remember to think about how you want to decorate your project at this step as well. Underglazes tend to show up best on buff or white clays and some glazes can change colour slightly when used over dark clays.
If none of these take your fancy you can buy small portions of coloured clay or colour the clay yourself using stain or oxide.
Step 5. Come instore to Mystery Creek Ceramics or buy online.
Remember to use the discount code 'Bulk Clay' to get $1 off per bag if you buy 10 or more 10kg bags.