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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15 thoughts on “Guest Post – Powdered Clay Anyone?

  1. Has anyone managed to build an oven from powdered clay in the UK? I’m desperate to build an oven but there is no “natural” clay anywhere near me..

  2. Thanks to all for all the great info! I am also building an oven. Right now, I’m finishing the base and collecting wine bottles for the insulation layer. As I am wont to do….I built the base way too big but am now going to add a grill on the side of the oven. It’s 72inx64in! I’ll have an outdoor kitchen by the time I’m done.
    I used cement blocks (8x8x16) and mortar for the base, 3 courses high. I’ve filled the centers of the blocks with rock and gravel and filled the center with fill (glacial till) and 5/8in minus gravel. I have an electric cement mixer! So I’ll probably try using it with the powdered clay. Thanks for posting about it! It will save me time doing my own experimenting. There’s a clay arts store on my way home from work so I’ve been stopping and picking up a bag every few days – I have a small car – one bag/trip.
    I have plans to build a structure to protect it from the Northwest rains we get all winter long. I hope to have it done by Christmas when we usually get a lot of family and friends visiting. I only get weekends to work on it and it’s MY project so the going has been slow, but a lot of fun. The guys I work with are amazed a woman would do this by herself, or even do it at all! I just tell them i have plenty of help from my blogging friends on the web.
    I have a blog too, and post pictures on my progress and recipes – love to cook.

    Thanks loads and I’ll keep checking back for more ideas.

    • Kudos to you, Jeanette! (What the heck is a kudo, anyway?).

      Great idea to add the grill. I have very good clay on my Pennsylvania farm, but when I finally get going on this, I may try the powdered clay. What part of the world are you in? Is there a brand name of clay to look for?

      I hope to have my oven firing in time for Thanksgiving turkey.
      Yrs,
      Bill Deutermann

  3. I too chose the powdered clay option, using Hawthorne High Fire and mason’s sand. I mixed in the ratio suggested in Simon’s book – 2 sand/1clay by volume – and adjusted water to meet the drop test – neither splat! nor crumble. My inner dome was a work of art – once fired, it cracked a bit but was easily patched. Several weeks of rainy weather later, I built the insulation layer and let it cure for three days, then built a fire that poured super-heated airwaves out of the chimney – very satisfying until…cracks started to develop in the insulation layer and – alas – smoke began to seep out of a length of fissure at the back. After the fire had burned out – and another week of rainy weather blown through – I built another fire in the dim hope that the oven fairy god mother had paid me a visit while I was making the rounds of suppliers asking for advice. S/he may have visited but certainly hadn’t fixed the problem. I’m now faced with the prospect of carving away chunks of insulation and patching the cracks with…(refractory cement?) or forging ahead with the final layer in my race against the onset of a New England winter. I’m writing in the hope that someone can offer a) an explanation of what may have gone wrong; b) reassurance that this is to be expected and won’t interfere with my use and enjoyment of the oven; and/or c) a solution to the problem. Please help!

    • Hi Dennis

      my guess would be that you have too much clay to sand. It is the clay that does most of the expansion and contraction and, as a result, tends to lead to cracking (imagine it was built from 100% clay and you can also imagine how much it would crack). Another blog visitor as recently suggested 3:2 (sand:clay) when using powdered clay. I have not used it and so all of my writing about it is from others expereinces. What would I do now? Carry on! Change the ratio for the final layer and pack it on thick around the cracked area. I seriously doubt it will affect the performance of your oven.

      I hope this helps.
      Simon

    • Hi Dennis:

      I made my oven using powdered clay in a 3:1 by weight, 3:2 by volume ratio of sand to clay.

      I just finished my first layer this past week and fired the oven to finish drying it out on Friday. I fired the oven for about 5 hours. I have observed a few small cracks, the widest of about 2 mm while the oven was hot. Once the oven cooled down it became a hairline crack, barely noticeable.

      My suggestion to you as far as patching would be to do so using the ratios as stated above, that is, 3:1 by weight or 3:2 by volume of sand to powdered clay. You may even want to consider going 4:1 sand to clay for the patching given that, as Simon points out, the higher concentration of clay is what got you in trouble in the first place.

      If I were you I would mix up a small batch of 4:1 by weight and 3:1 by weight to judge by feel which one you think would work best.

      Like you, I too am trying to beat the onset of Winter on the Eastern Seaboard of the US, though our winters pale in comparison to yours!

      Good luck!

  4. Yesterday I finally got to finish the first layer of the dome.

    For anyone wishing to know how much weight of each item, here goes.

    Our oven is 16 inches (40 cm) high and has an internal diameter of 27 inches (64cm).

    Sand for the form took 320 lbs (145kg.)

    I used a 3:2 sand to powdered clay ratio, by volume. In terms of weight, this broke down to 60 lbs (27.3 kg) sand to 20 lbs (9.1 kg) powdered Redart brand clay. As you can see, by weight it is more of a 3:1 ratio.

    It took three batches to complete the first layer. The walls are 3 inches thick (7.5 cm) and we used every single scrap of mix, there was literally none left over!

    Now, we wait for it to dry so as to cut the door and continue.

  5. most of these items are sold by weight, so 3:1 by volume means how much by weight? if the first layer uses 100 lbs of sand, how much dry clay would that require?

    • Hmm, you got me there. Since I have only been able to finish the base and the sub-floor, I cannot give you exact figures but here goes:

      The sub floor you see in the picture above is roughly 40x40x4 inches deep. In filling that void I used about 3/4 of a 50lb bag of clay plus approximately 60lb. of builders sand (not the playground or paver stuff).

      Please note that in your comment you mention 100LB of sand for the first layer. I have not discussed how much sand and clay I will need for any layer in my yet to be finished oven. However, I have a 40×40 inch area to lay with Fire brick, so my internal diameter will be on the larger side of things. That being said, I purchased 10 50lb sacks of clay just to be safe. In looking for a clay supplier locally, it turns out that they all buy from where I got mine, so I think I’ll be able to sell whatever I have left over to one of several shops at my cost, whilst keeping a bit for eventual repairs and whatnot (or bits and bobs as our host is wont to say!)

      Now that we are finally getting a long stretch of decent weather without rain, it seems I will be able to advance with the dome and first layer this week, thank God!

      I will keep track of all pertinent info such as how much weight and volume of sand and clay I use, and internal and external diameter of the oven’s first layer so that you have an idea of what to expect!

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