Guest Post – Powdered Clay Anyone?

A first for The Clay Oven, a guest post by Robert Nasser from Washington D.C. Robert writes about his experience of building a clay oven using powdered rather than ‘plastic’ clay. A useful and interesting alternative method, particularly exciting for the lazy-puddlers amongst us!

When building a clay oven one has many choices as far as clay is concerned. You can dig up your yard (or your neighbour’s yard!), buy moist clay from a pottery supply or you can buy dry, powdered clay.

Even though the soil where I live is high in clay content, and even though Simon has done an excellent job in describing the puddling technique for mixing moist clay/soil and sand, there are those who like me, look for the easiest way possible because, well, because we’re lazy!

In reading Simon’s blog and doing research on other Internet sites and books, I slowly formed an opinion about how I would go about building my own oven. I settled on using dry powdered clay for the following reasons:

1)      I believe a consistent mix of clay and sand can be achieve.

2)      I can do so with less effort than puddling moist clay or soil with sand.

3)      I will not have to worry about stones and rocks as I would if I dug up my garden. Rocks and stones need to be taken out of any soil you dig up, as they become focal points for weaknesses leading to cracks! It is a fact of life that all clay ovens will develop cracks, I posit that we don’t need to encourage them!

So with that in mind, I decided to experiment before taking the plunge into buying tons of powdered clay.

The first thing I did was to make three batches of clay and sand with different ratios of sand to clay. For this purpose I decided to use what is known in the US as builder’s sand. This is sand of varying size granules and of irregular shape. It is not playground sand, nor is it “paver” sand which are sieved and sorted so that they are very smooth and almost powdery. Some paver sands are also treated with polymers and the like, and I have no desire to find out how this will affect the mix.

The clay I used is a Redart brand terracotta. It has no grog or other additives, it is simply powdered clay with a talcum powder consistency.

I prepared a batch of 3 parts by volume sand to one part clay, 3 to 2 and 2 to 1 as well. The idea being to find which ratio shrank least while conserving the desired qualities of strength, workability and able to pass the “drop test”.

I baked the different bricks at 200 F (93 C) for about 6 hours in my kitchen, and left others out to dry inside the house due to the unusual amount of rainy days around here (two weeks of rain, which is very unusual!) In doing so I came to the following conclusions:

The 3 parts sand to two part clay ratio gave the best performance, given that there was hardly any measurable shrinkage. I observed 1/16th of an inch over 3 inches (1.6 mm over 76.2 mm) (roughly 2%)  and it remained very workable when mixing and when forming. It passes the drop test and hardens satisfactorily.

The other ratios either shrank more, the 2:1 shrinking almost 6 %, or did not do well on the drop test as was the case for the 3:1 ratio.

So, now that I had determined the best ratio of sand to clay, I went about mixing a bigger batch that would approximate the batches I would need when it came time to build the oven itself.

For this purpose, I found it easier to mix it up in a mortar pan. These are made of plastic and are sold at home improvement stores here in the US. They are rectangular bins where you can mix the equivalent of 50 kg of cement or mortar. To mix I used a hoe, a shovel and my hands! I have read where some have used a cement mixer for this, but I found that my way worked for me and I didn’t have to rent a mixer! If you do have one handy, try it, but I really don’t see the need to go out of your way to have one!

The procedure is really very simple. Lay out the sand in the pan, put the clay evenly over it and mix well. The key is how moist is the sand you are working with! In my particular case the sand I was using was pretty moist to begin with (2 weeks of rain will do that!).  Using the hoe or shovel, mix the clay and sand very well, until you are sure that there are no pockets of either, nor any large clumps. If the sand you are working with is very dry, then wet it until it is fairly moist, as if building a sand castle at the shore.

As you mix the sand and clay you will notice it achieves the consistency and look of dry oatmeal.

Once the mix is ready, pour water a little at a time and mix. You do not want to pour too much at once, or you will find yourself adding sand and clay so go slowly with this!

I start out using the hoe and shovel, but eventually you will see the point where you need to use your hands to ensure consistency. Feel free to use your feet if you think that works better and you wish to save money on a pedicure!

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