Powdered fire clay

[dellboy78]

Just about to buy some powdered fire clay. If I buy 50kg of it, then mix anything from 2-4 parts sand to 1 part clay, and then add water I could potentially end up with anything from 150-250kg of ‘finished’ building clay. Is that enough to build an 80cm diameter oven do you think?

Would rather buy too much than not enough but don’t want to go too mad!

Thanks

[downunderdave]

Apr 13, 2020 16:28:08 GMT dellboy78 said:

Just about to buy some powdered fire clay. If I buy 50kg of it, then mix anything from 2-4 parts sand to 1 part clay, and then add water I could potentially end up with anything from 150-250kg of ‘finished’ building clay. Is that enough to build an 80cm diameter oven do you think?

Would rather buy too much than not enough but don’t want to go too mad!

Thanks

Not knowing the size of your oven or how thick you are planning to make the dome, it is not possible to estimate the volume of material required. Use the formula for volume of a sphere, subtracting inner radius volume from outer radius volume, then divide it by 2 because it’s a hemisphere. Use 4/3 x Pi x r3 The cob mix is usually 1 part powdered clay to 2 parts sand by volume. The density of sand varies a bit but is around 1450 g/L and the density of powdered clay is 742 g/L The cob mix is generally reinforced with chopped straw added during mixing.
Hope this helps you.

[dellboy78]

Hi,

Thanks for that info. I have done a few rough sums....
Whether they are correct or not is another question!

So internal diameter of oven will be around 79cm with the thermal layer being around 7.5cm thick.

Based on that I think the volume of the thermal layer will be around 88.5L

Two thirds of that (59L) will be sand and the other third (29.5L) will be clay. (This is only rough as I understand that due to clay particles being so small they will fill the gaps between the sand so 1 litre of clay and 2 litres of sand will not necessarily give me 3 litres of dry mix)

Based on your density figures then, I think I will need around 22kg of clay and 86kg of sand for the first layer. Give or take!

As the bags of powdered clay come in 25kg I reckon if I get 3 bags that should give me more than enough for the thermal layer, the insulation layer and a thin ‘finish’ layer will plenty left over for any repairs etc I may need to carry out in the future.

[downunderdave]

My calculation comes out at 90 L (much the same). The amount not required for the oven mouth is around the same as is required for the extra needed to create the vertical form of the mouth so they cancel each other out. You will need extra to create the flue gallery though, if you’re having one. This is best cast after the oven casting has been done, so you can create a rebate for a door.

[dellboy78]

Got my door arch planned out at last, does it look ok? Is the powdered clay and sand gonna hold this thing together ok? Or should I use something else...