11. Clay Clay Clay!

After several weeks of receiving many questions from Clayoven visitors regarding where to find clay and specifically what type of clay one should buy from potters suppliers, I decided to ask an expert.

Sue works at Pottery Ceramic Services (the Kiln Engineer – a supplier of all things pottery related based in Fordingbridge, Hampshire,UK) and knows a lot about pottery clay (useful in her profession I guess!).  Last week I cheekily asked her if she would like to write a small post for the blog about different types of pottery clay and she kindly said yes.  This is what she had to say:

There are 2 main varieties of clay, Earthenware or Stoneware.  This divide is depending on firing temperatures. Earthenware clay is fired in a kiln at a lower optimum temperature (maximum of approximately 1160ºC) and Stoneware clay is fired at a much higher optimum temperature (maximum approximately 1280ºC)

For building a Clay/Pizza oven, although the clay is not going to be kiln fired, the heat from the actual firing is enough to heat the clay right the way through, so Earthenware clay is the usual clay to purchase.

Prepared clay bought from a pottery supplier will be ready to use, all impurities such as stones etc. will have already been extracted.  The last thing a production potter wants to do is mess around with the clay in the first place making it fit for use! So it will have gone through a process of pugging and will be ready to use straight from the bag.
Pottery clay comes in many different colours and grades from a very fine clay for detailed sculpture work to what is known as a heavily grogged (particles of already fired clay) clay which is suitable for hand-building, and this is what I would suggest for the Clay Ovens.  
Grogged Terracotta Clay is mostly sold for the making of Clay Ovens as it has about 10% grog and when the clay has dried out, and has been fired through, is a lovely warm colour. Extra grog or sand may be added to the clay if required, although many people use it just as it is with good results [I would still reccommend that you use a sand/clay mixture for your oven at the normal ratio of 2:1 sand:clay, grogged or not! - Simon].

Clay is usually sold in 12.5KG packs and depending on the size of Oven required may take 10 – 15 bags to complete the project [I cannot verify this becuase firstly I didn't buy clay for my oven and secondly I never weighed out the actual amounts I used - Simon].

So now you know.  How fabulous is that – thanks so much Sue!

Of course, in the interests of fairness I would add that there are other pottery suppliers out there who will also supply you with potters clay.  However, I’m sure none of them will be as super friendly as Sue and the team at Pottery Ceramic Services.  In return for her aticle I promised Sue a gratuatous plug, so if haven’t done so already, here you go!

Contact details:

POTTERY CERAMIC SERVICES,
UNIT 2 ARCH FARM INDUSTRIAL ESTATE,
WITSBURY ROAD,
FORDINGBRIDGE,
HAMPSHIRE. SP6 1NQ.
TELEPHONE NUMBER 01425 655540
EMAIL ADDRESS sue@processcontrolsystems.co.uk
WEBSITEthekilnengineer.com

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66 thoughts on “11. Clay Clay Clay!

  1. I did my first build with the same clay powder as you want to use.my first mix was 1/1 with sand and clay. the whole thing colapsed when I removed the sandform. later i used 3 parts clay and 1 part sand and I got huge cracks and a layer of the back inside of the oven falling in. So the 2 buy 1 might be better. I am thinking about doing a brick build this time.
    The other problem with this mix is that we have a very high humidity here in PA. The door arch, or better the clay I used between the bricks is disinegrating and the arch is slowly caving in.

    • It’s worth mentioning here that I have not used powdered clay in the mix myself. Clearly ratios, water volumes etc are different when using powder compared with traditional wet clay.

      • Simon:

        What I am finding in using the dry clay how the amounts translate vs. wet for ordering purposes.

        I am finding I need very little water to get the clay/sand mix to combine in such a way as to pass the drop test and to form bricks that harden nicely.

        When making a slip, using the dry clay, it takes roughly a 1:1 ratio in order to get slip that has the consistency of heavy cream (AKA whipping cream)

        Since the sand I am mixing the clay with is very moist, it is taking very little water to achieve a satisfactory consistency. I would guess maybe 10% water at the most!

      • Hi Roberto

        yes I was always sure that dry clay would actually make the mixing process much much easier. I never promoted the idea too heavily on here because I had no idea what the mixture quantities where. It would be great if you could share the quantities. Maybe you would like to write a guest blog post about it?

        Simon

      • Thank you Simon! I certainly would like to write a guest post.

        Shoot me an email robert AT dinnerdone dot com

        I would like you to edit the post before posting it!

    • HI WKH:

      I’m in the Washington DC area, so our weather is likely similar. It certainly has been very humid around here the last couple of weeks!

      If you used 3 parts clay to 1 part sand, it is no surprise you are seeing cracks and that the oven is coming apart. It sounds counter-intuitive but you actually want more sand than clay, whether using wet or dry clay. Likely the reverse would have stood you in good stead! Clay is pretty elastic and subject to expansion and contraction. Sand helps form a matrix that keeps the elasticity down.

      Given that it seems like your oven is falling apart, I don’t blame you for wanting to go the brick route.

      If you are near Pittsburgh, you will have no shortage of places to find great variety of firebrick, given that city’s history with steel!

  2. I finally finished my oven thanks to your great blog, and i plan to do a writeup of my project to share with others too. I just had a comment/question on the subject of clay and cracking.

    I used heavily grogged and sanded clay, tried a 2:1 sand ratio, also tried a 3:1 sand ratio, tried a slip with perlite, dried slow, dried fast, tried chickenwire as internal support…and in all cases there were cracks cracks cracks. I tried covering these cracks or (as someone suggested) dig a trench through the crack, get it wet, and fill the crack…it just cracked again, in the same spot or a new spot.

    I spent a lot of time on the first layer, firing and smoothing, getting rid of cracks, but the inside again has many small cracks in it. I am not the least bit concerned with the cracks, functionally or aesthetically. I just want to do the correct thing with regards to keeping my oven usable for many years.

    Something tells me (based on my limited experience) that these cracks are not only a fact of clay ovens, but are a desired and necessary effect. I have my doubts that any combination of materials will yield a crack-free oven. And (like tectonic plates) may allow for contraction and expansion as the oven is used. And the best we can do is have some extra clay on-hand to maintain the oven.

    Any comments or contradictions are always appreciated.

    • Hi Adam:

      I am just finishing my plinth and am getting ready to lay my hearth and start the first dome.

      While I was doing this I experimented with different clay to sand ratios (plus I decided to use powdered store bought clay).

      Since it has been raining here where I live, almost nonstop it seems, for the last couple of weeks I tried speeding up the process regarding the clay/sand mix. So I made different “bricks” and was trying to get them to dry so I could measure the shrinkage. After about 10 days they still had not dried completely (they were outside, under cover).

      I moved them inside and took some to dry in my oven and others I simply left lying around the kitchen. I set the oven for 200 F and placed the bricks inside for about 6 hours. They all shrank some, but the ones with less sand shrank the most and also cracked.

      The ones I did not put in the oven took about 4 days to dry, hardly shrank and did not crack. As expected, the higher the sand to clay ratio the lower the shrink percentage. All bricks, both oven dried and air dried came from the same batches (i.e 2:1, 3:1 and 4:1 batches each had oven and air dried bricks)

      My conclusions, both from observation and from what I have read up on different fora is as follows:

      A higher ratio of sand to clay produces less shrinkage. I found that the 3:1 ratio worked best for the clay and sand I am going to use
      (powdered, Red Art brand clay made in USA + Quickrete builders sand)

      A slow curing of each stage reduces cracking, which is a product of shrinkage + time to dry. The faster you dry things, the faster they shrink, increasing the likelihood of cracks.

      I plan to cure the first dome for about a week by leaving candles inside along with the occasional small fire. The idea being to do so slowly and to try to prevent cracks.

      Let’s face it, cracks are going to be a part of our ovens. We can control, however, how and when they happen.

      • i agree with you, and find your experiment fascinating with different sand-clay ratios. but i think low temperatures like this are an unfair evaluation. another blogger said they were happy with no cracks since they got their oven to 300 degrees.

        when i fired my oven, i used an infared thermometer and observed temperatures well over 1000 degrees. so we are not just talking about shrinkage here, but expansion and contraction. think of making the oven totally out of cement, where there would be very little shrinkage, but also very little ability to expand and contract. i believe there would be more severe cracking than an oven made of clay, which has more plasticity.

        the other issue is that your ratio when using powedered clay is going to be different than using wet clay, and of course different if your ratio is using weight or volume.

        so while i’ve read about a great deal of oven designs and mixtures, we are talking about extreme temperature differences and i have my doubts about our ability as builders to reduce cracks for such a temperature change.

      • Unless you build a brick and mortar oven it will crack. The important thing is that it is unlikely to be catastrophic so son’t worry about it!

        Simon

      • Absolutely. I really like your plate-tectonics analogy (more so because I am a Geology graduate!). These ovens need to expand and contract and the only way a semi-rigid structure like this is going to accommodate this is through cracking.

        LIVE WITH AND LOVE THE CRACKS!

        Simon :-)

      • Oh, and for the record I went with volume vs. weight in regards to the use of powdered clay.

        So three measures of sand to one measure of clay seemed to work best. Since the sand was moist out of the bag, the amount of water required is minimal.

        I found it pretty easy to mix. I just used a large plastic bin such as they sell here in the US to mix mortar in.

        I put the sand in, spread the clay on top and mixed well using a shovel. Once I was certain that the mix was uniform, I added just enough water to make it all stick together. Drop tested and moved on!

    • @ Adam:

      1000 degrees? Centigrade? Holy cow!

      I am expecting temps of 300-400 Centigrade, which is plenty fine for pizza and breads, but 1000 C? Wow

      Given the materials, you should certainly expect cracks, that much heat and the corresponding cooling; expansion and contraction is huge.
      Cracks?
      I guess you’ll just have to get used to them! :-)

      • No, I am in the US and have an infared thermometer that reads up to 1000F which is 538C :) I was reading on another of Simon’s pages that if you use powdered clay, you will use a higher sand to clay ratio than when using wet clay, so your using 3:1 is not surprising. I only had access to wet clay.

      • Phew! You had me there for a while!

        Still 1000F is pretty awesome!

        Looks like decent weather for the remainder of the week, so I look forward to getting the first layer done by Sunday evening.

        It has rained so much around here lately, I feel like I’m in England, or Seattle!

  3. Hey Simon,
    I’m building a clay oven and I’m having a great deal of trouble finding a good source of clay. I intend to buy, but finding a potter’s supply shop has been a huge chore. The store you gratuitously plugged is great, but it’s a bit too far away, as I don’t want to have to drive several hours to get my clay. Do you-or anyone else–know if there is a pottery supply store in Surrey? I would be extremely grateful.

      • Well… yes. No. Sort of. The deal is, I don’t exactly know how much clay I will need. This very post mentions as little as 4 bags and as many as 14. With that in mind, ordering clay over the internet is difficult, because if I don’t get enough, I can’t quite pop over to the store and get some more–we’re talking about another 5 days of waiting, another shipping surcharge, and an amount of clay that may be far too much or far too little for what I need. So overall, the short answer is “Yes, technically,” but the benefits of having a local shop are obvious.

        About googling: You would not believe how difficult it is to find a potter’s supply shop in this country! Painstakingly combing through yell.com, google, trade websites, etc. have yielded very few results in general, and none (that I can find) in my area i.e. Surrey.

      • Wait! factual error. The wildly varying quantity estimates are *not* on this post–in fact, I can no longer find them. However, the point remains–we don’t exactly know how much clay to use, because everyone mixes theirs differently and makes different-sized ovens. Thought it should be rectified.
        -Sam

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