Red Clay Bodies continued

A quick recap just in case you haven’t read the last two blogs….

I am interested in the intersection of English slipware, typically made from terracotta coloured, earthenware clay bodies and Korean Buncheong ware – again, a freely applied slip over a dark clay body, but fired to stoneware temperatures.

For the last couple of years I have been searching for a red stoneware clay body that would allow me to express these ideas. However, there have been difficulties with the clay creating bloats or blisters under the slip and so rendering the pots unusable. I have sought out both the reasons and the solution to this problem. So far, the most persuasive reason is that Red Iron Oxide converts to a different form during the glaze firing process and gives off oxygen in the reaction which gets trapped under the slip. It doesn’t happen in a reduction firing, but I fire in electric kilns where the process is in oxidation (i.e. the presence of oxygen).

I will return to red iron oxide later but of course, there were other theories about why this was happening; perhaps it wasn’t to to with iron oxide, but the decomposition of organic matter, carbon or sulphur during the biscuit firing*. Clay bodies contain all of these materials and chemical reactions take place up to about 900⁰C to burn them out. In the case of organic or vegetable matter they give off a smell, and with carbon and sulphur their burning off gives off carbon or sulphur monoxide, dioxide or trioxide.  

 I've tested various biscuit firing schedules, mainly using a slower ramp up to the top temperature of 1000° C and holding a ‘soak’ temperature for an hour.  The soak at 650°C  gives time for the heat to penetrate the work and consequently the chemical reactions in the clay body will be completed thoroughly.

For those interested, an example of this type of biscuit firing is as follows:

25 ⁰C per hour to 100⁰C (candling the work, to make sure no water is present)

75⁰C per hour to 650⁰C

60 minutes soak

75⁰C per hour to 1000⁰C

 At the moment, my conclusion is that this doesn't make any difference to the finished piece and so my current biscuit firing schedule is as follows:

25 ⁰C per hour to 100⁰C (candling the work, to make sure no water is present)

75⁰C per hour to 1000⁰C

 One of the happy side effects of all this work with my new clay body is that this slow biscuit firing brings out the speckle in the clay that I use when I make my Midsummer Flowers collection, and this is the reason why I have kept using such a slow firing schedule.

So, back to red iron oxide and how to solve the bloating problem. In my observations I have noticed that those vessels that are larger and therefore have thicker walls tend to blister or bloat more readily. I have been practising throwing larger cylinders and bowls where the wall thickness at the base is consistently narrow. This is no mean feat, but where I can make it happen, there are no blisters. For many, the solution would be turning the pots once the clay is leather hard. However, because I want to retain the energy of the pot in the final piece, I have chosen to not turn some of the forms in my pottery – it is what is made in that moment that excites me.

And secondly, enclosed forms do appear to influence the development of blisters. This is problematic for me as I rather love the vases and jugs that I have been making which have a neck! The explanation for these two observations is that there is more Red Iron Oxide in thicker walls and there is less air around in a more enclosed form, to allow for the chemical reaction to take place, so the walls blister.

In the end my current solution has been to reluctantly stop using the red clay for larger, enclosed vessels. With these I now throw them using a dark grey coloured clay and then two slips to coat the pot – firstly, a black slip and then the usual white slip on top of this. Despite the sense of compromise and the longer time it takes to make the pot, this new process has made me feel much happier to know that when I open the kiln it is more likely to be full of successful, saleable pottery than those that have to be added to the shard pile.

I will be continuing to experiment with other slip formulations to try to find a way to eliminate the additional process of a second slip. I have realised that I will forever be experimenting with something or other, as this is just in my nature to want to push forward into new areas.

My next blog post will be about exactly this, creating my own clay body.

*biscuit firing. This is the first firing of the pot which usually goes up to about 1000 degrees Celsius, and means that the pot can still take on a glaze as it remains porous, but is thoroughly dry and so definitely won’t explode in the glaze firing, causing shards of glass to coat the kiln and make it useless.