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 Briar Wheels (a supplier of all things pottery related based in Fordingbridge, Hampshire) 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 handbuilding and 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 Briar Wheels. In return for her aticle I promised Sue a gratuatous plug, so if haven’t done so already, here you go!
Briar Wheels super friendly potters suppliers
Several points
(1) A big concrete paving slab seems an easier way to make the oven base. Probably no more environmentally nasty than commercial clay bricks.
(2) If you advise adding sand, is there any point in paying (a little) extra for the grogging in the clay?
(3) If you want to make the outer surface more resilient to weather, why not use mosaic tiles – perhaps broken fragments of tiles or purpose made mosaics? This would give a largely glazed outside, and might only need repointing to stay water resistant. You might have to wait until the oven had been fired a couple of times to allow the rest of the clay to dry.
(4) The oven we saw at the Henham Steam Fair (cooking pizzas for the visitors) did not have a chimney – is the chimney really necessary?
(5) If you do need the chimney, then why not also an air vent at the back to help get the oven fired up? It could then be blocked off during normal cooking.
(6) How about using non-inflamable insulation (such as vermiculite – made from rocks) in the insulation layer to avoid the problems with the wood chips catching fire?
Cheers
David
Thanks for the input David. My thoughts:
(1) yes I have seen paving slabs used so go ahead. Not sure how the production of concrete compares to the production of bricks in terms of their respective environmental impacts.
(2) Grogging, which for those who don’t know, is the process of adding small particles of fired clay to wet clay – normally for pottery making purposes. If you are buying clay from a pottery suppliers you DO NOT need to buy grogged clay. As David infers, we add sand to the mix anyway and the oven walls are never “fired” as such so the grogg has no effect whatsoever.
(3) Again I have seen ovens with mosaic patterned outer layers. The thing to remember though is that these ovens do crack and, as a result, you might find that your tiles end-up falling off. Worth a try though!
(4) You need an entrance for air to get in and a different one for air to escape otherwise your fire will go out!
(5) An air vent at the back would be fine.
(6) Vermiculite seems like a sensible idea.
All the best
Simon
Please dont shout – but if the clay is prone to cracking could it not be substituted with a fibre reinforced concrete mix? – I’d have thought it would have the same heat retention proprties and if made with two layers you could use a glass fibre insulating layer sandwich between …
glad to hear other vies or thoughts
Hi Derek. You can of course use concrete, cement, brick or any other materials to make an oven. My main choice for using the clay/sand mix is an environmental one. The cement industry is globally responsible for pumping out vast quantities of CO2 (over 5% of global totals – http://www.guardian.co.uk/environment/2007/oct/12/climatechange). As for glass fibre – I believe it is a carcinogen and no doubt also has questionable manufacture processes associated with it (pollution etc).
Ovens like mine have been built using the same materials and techniques for millennia and, yes they do crack and eventually fall apart, but one can always build another one – that’s part of the fun!.
Cheers
Simon
can you glaze over the final layer ,would this strenghten the oven? would it minimise cracks?or the opposite.if so what kind of glaze?
I can’t think of anything that would be suitable Danny. A glaze, in the traditional pottery sense of the word, would not work because the outside of the oven never reaches a high enough temperature. I have seen ovens with a white finish which could be lime mortar. I prefer the au naturel look though personally.
Cheers
Simon
cheers simon ,one more tought could you fire the outside with a fire torch to give that pottery glazed look or would this still not be hot enough?
Hi Simon, thanks for the blog, a great resource you’ve created here – the oven looks superb and I love the way that you’ve stuck largely with organic materials.
I’ve been working on a portable contraption for the back garden in Sydney – still a week or two away from completion but can’t wait to fire it up for real.
Check it out if you have a chance, would appreciate your feedback.
All the best
I love the blog and I love the oven Simon – brilliant! I have been thinking about building a portable version too recently so I will be watching the final installments of your blog with interest. I’ll also take you up on the offer of a pint when I’m next in Sydney!
Great work!
Simon
Hi There
I have just built a brick oven – used ‘Your brick oven Building it and baking in it by Russell Jeavons – however I think the ceiling is a bit too high in relation to the door – I though I would line the inside with a 2 inch layer of clay – can anyone advise what clay to use to do this? And where I might get it from?
I can send a picture of it…
Thanks
Richard
Hi Richard why don’t you try contacting “Blair Wheels” – the link is above.
Good luck
Simon
could i use the same principle to build a tandoor oven?
Hi Gill I can’t think that this would work for a tandoor – at least not a traditional shaped tandoor oven.
Simon
Hi there,
I’ve just got my clay ready to build my oven this weekend, but other things got in the way so next weekend it must be for construction.
I went to Anglia Clay Supplies for my clay (website is http://www.angliaclaysupplies.co.uk) and collected it. Get it delivered – much easier! They recommended their Red Earthenware clay which is only £7 odd a 25 kg bag. Lady there was most helpful and has built her own oven, and knows what she is about.
Cant’ wait to get it built, and will post about our fist pizza(s).
Roger
grogged i mean, predictive text, sorry
To all,
I purchased 16 bags (200kg) or the 10% frogged terracotta clay from Sue at Briars Wheels. It cost me £78.
I have only done the 1st layer, when I have completed the oven I will let you know how much clay I had left or needed to get. My oven has the same measurements as Simon’s. The colour once dried is the same colour as a bar of dairy milk chocolate.
Just waiting for a dry day to do my entrance.
Nic
Hi Danny I doubt it would work. Pottery kilns reach temperatures well in excess of 1000 degrees C. I’m no expert though so by all means give it a go and let us know.
Cheers
Simon