Sticky or dry?

Clay Powder*

A question that I commonly get asked on the clay oven blog relates to the process of mixing the clay:sand mixture used to build these ovens.  I learnt how to prepare this mixture by using wet clay and mixing it into the sand using an old fashioned technique called puddling (I believe the Roman’s used this same technique for mixing material for their villa floors – they used things like dung, soil and cows blood though!!).

You will find a post on how this is done here:

https://clayoven.wordpress.com/2008/09/04/3-the-clay-sand-mixture-and-puddling-technique/

Puddling is effective but rather laborious and time consuming. Many suggestions have been made by visitors to this blog from using cement mixers to industrial hand “blenders”.  The problem is that clay and sand don’t really like to mix unless they are ground together using the puddling technique.

One commenter suggested drying out the clay first, grinding it and then adding it to dry sand and water in a cement mixer.  Technically a potentially useful solution but in practice, drying and grinding clay to powder would be a nightmare and would likely take much longer than getting your feet dirty, puddling.

This week I have had a few conversations with blog visitors Steve O’Conner and Dori who inform me that one can purchase bags of dry, powdered clay.  Dori said:

“Ive actually used the powdered stuff to build my oven – bags of FireClay (which was also called mortar clay in my store). We first did the tarp and dance method where we poured all of our ingredients on a tarp and stomped on them, but eventually used a cement mixer which was much faster. What ive learned from the powdered stuff is that you need a MUCH higher ratio of sand:clay then the lumpy stuff. Pure clay in powdered form shrinks quite a bit. Other then that, it has worked just fine for me.”

Well I never knew this but it sounds like a great alternative to puddling to me!  I will try some experiments later in the year to see what ratios one needs exactly.

In the mean time, if you try it out, please be sure to share the details on here.  Thanks for the info chaps!

Simon

*Image taken from http://www.starwest-botanicals.com/images/D/210106-51.jpg

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8 thoughts on “Sticky or dry?

  1. After a month of drying, I ran a small fire in my new tandoor, made with fireclay, sand and cut rice straw. Then this weekend made a real fire to see how the tandoor will fare.

    First of all, burning wood in it is a joy, the bottom vent and chimney shape make the wood burn very cleanly and vigorously. After an hour, the soot disappeared from the inner walls, a sure sign of temperature in excess of 300*C being achieved. I have a IR gun thermometer that goes to 550*C; much to my surprise, the bottom part of the wall read “H” (i.e. more than the max of 550*C)!

    The interesting aspect is that the walls have developed no crack whatsoever, despite the very high temperature achieved. My mix was 1 part fireclay to 3 parts sand, with some cut straw added (not too much).

    Long story short, fireclay powder and sand is a method that works well if you don’t know where to harvest your own clay.

  2. Thank you for this information on powdered fire clay or motar clay fir building an earth oven. You mention that the sand/clay ratio needs to be much higher. Would it be possible to give an approximate ratio? Also, I can find clay with sand already mixed in, do you know anything about this?
    Thank you,
    Marilyn

    • I’m afraid I can’y help with exact ratios Marilyn – I have not used it. Some of the other people on here may be able to help. Never heard of the ready-mixed clay:sand but I doubt it has enough sand in it.

      Best of luck

      Simon

  3. I found some sandy red clay 3ft under my lawn. I didn’t add any additional sand… thins may turn out to be a massive mistake but, we just completed our insulation layer and after a weeks drying, we should be able to tell…
    We found puddling fun, too messy, you end up 3 inches taller as most of the clay sticks to you shoes/wellies.
    We found a much more efficient alternative method using a tarp… just as back breaking, but a lot less messy…

  4. Hey,
    Great blog – I’m about to embark on my oven. We live in a very sandy area, miles from any clay, so I am tempted by fireclay!

    Do you have any more information on the ratio for sand:fireclay? Will be having a go with a mortar mixer (a giant hand held food blender type thing!)

    • Thanks Andy! Any old clay will do, the cheaper the better. Keep the ratios the same 2:1. I have used a mortar or plaster mixer for mixing the clay slip for the insulation layer. Not sure it’ll work for the sand clay mix but worth a go. Good luck!

      Simon

      • I’ve successfully used a mortar mixer to build the first layer – which is now dry and been fired a couple of times (http://www.facebook.com/media/set/fbx/?set=a.10150220294082165.350457.731657164)

        I got blue cambria clay from a friends brick pit near Cambridge, which was still pretty wet and did a 1:2 mix with good old Wickes builders sand.

        A bucket of sand and half a bucket of clay took 5 mins to mix with a heavy duty mixer paddle from screwfix attached to an old 600W powerdevil drill. Building with that allowed the drill to cool down ready for the next batch.

        I did the drop test fine but my mix must have been moister to allow the mixing, so it took a lot longer to dry – overnight was not long enough – my first go collapsed. Leaving it a couple of days was fine and a couple of firings later I’m ready for the insulation layer!!!

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