Alternative hearth

[gracie]

sorry to be posting so many questions in quick succession, I'm keen to press on but don't want to commit to buying materials until I have it all straight in my head!
i have been wondering whether I could use a kiln shelf for the hearth rather than the arduous process of cutting fire bricks into a circle. The downside is the maximum diameter I have found is just shy of 600mm and its only 19 MM. thick. 
Two questions then...can I put a layer of clay/ sand mix under the shelf, on top of the insulation layer to increase the thermal mass of the bass and can I extend this layer to increase the diameter to 700mm? All of this assumes that kiln shelves are made of refractory material rather than insulating material. Any ideas?

[oblertone]

Hi Gracie, I'm not sure about the refractory qualities of the kiln floor, perhaps the manufacturer could assist; but before you commit to a smaller oven dictated by the disc consider the alternatives. It is possible to build your oven on top of a pre laid floor, and while it's not as thermally efficient as a 'floor in dome' design it works perfectly well. Have a look through my build thread below for an example of what I mean.

Paul

[gracie]

Hi Paul,
Thanks, so what you're saying is that the hearth doesn't need to be cut into a circle? How does this affect the insulation? Wouldn't the heat transfer along to the area of hearth sitting outside of the insulated dome? ...and does that actually matter?
I have asked the manufacturer about the kiln shelf so I'll report back.

[oblertone]

On my build I laid a flat hearth and built a dome on it; although there are some other limitations with my specific build it works perfectly well for pizza and has enough retained heat for bread next day followed by a roast.

[downunderdave]

Apr 14, 2016 6:43:51 GMT gracie said:

i have been wondering whether I could use a kiln shelf for the hearth rather than the arduous process of cutting fire bricks into a circle. The downside is the maximum diameter I have found is just shy of 600mm and its only 19 MM. thick. 
Two questions then...can I put a layer of clay/ sand mix under the shelf, on top of the insulation layer to increase the thermal mass of the bass and can I extend this layer to increase the diameter to 700mm? All of this assumes that kiln shelves are made of refractory material rather than insulating material. Any ideas?


Kiln shelves are very strong, refractory, dense and not insulating. However at only 19 mm thick I'm not sure how they will withstand the uneven heat from a fire sitting on top of it, as the outsides will be way cooler. In general large pieces of any refractory material are more subject to cracking. This is precisely why bricks are preferred because the gaps between them can act as expansion joints to prevent stresses caused by uneven heating.kiln shelves are also very expensive, at least where I come from, but that is probably mainly because they are imported from UK or US. If you can source yours locally maybe you can get a decent price.

[davenott]

I'd use firebricks. If you want to increase your thermal mass, lay them side on rather than flat.
If you don't want the hassle of cutting them, maybe just lay a square hearth floor.
I found I could cut them relatively easy, by plunging them in water first, then cutting them with 9 inch angle grinder. I bought the Titan 9" angle grinder from Wickes, which is a well made unit, but I still destroyed mine as I finished my large oven (brick dust eventually kills them) but they have a 3 year guarantee and Screwfix were good and gave me a new one even though I had lost my receipt. You need a diamond blade, my blade did the whole oven, but I eventually destroyed that by cutting kerbstones.
I think it would take about 2 hours maximum of cutting to cut your round hearth of firebricks, and that is including resting the angle grinder. I cut for a few minutes and then rest it a minute to cool.
The angle grinder is heavy and needs to be used with respect and care as well as safety gear. (Mask, goggles, gloves and heavy clothes and foot ware.) You will get lots of dust. If not experienced with using angle grinder consider getting someone to help you who is experienced.

[downunderdave]

Brick dust is dangerous stuff if inhaled because it is tiny shards of fired clay which damage the tiny hairlike things in your lungs. This is the main reason that brick cutting machines have a water feed to reduce particles becoming airborne. Wetting the bricks will partially help and a dust particle mask may help a little too, should really have a respirator mask. The odd one may be ok, but cutting a lot of bricks with an angle grinder is foolhardy IMO.

[davenott]

Apr 14, 2016 21:37:36 GMT downunderdave said:

Brick dust is dangerous stuff if inhaled because it is tiny shards of fired clay which damage the tiny hairlike things in your lungs. This is the main reason that brick cutting machines have a water feed to reduce particles becoming airborne. Wetting the bricks will partially help and a dust particle mask may help a little too, should really have a respirator mask. The odd one may be ok, but cutting a lot of bricks with an angle grinder is foolhardy IMO.

I agree.
I use a tight fitting respirator mask, always cut outside and change the filters regularily. You should be able to blow your nose afterwards and see no discolouration from dust.

[gracie]

Thank you all. I think I will lay a hearth in firebrick either in a rectangle or as close to a circle as I can without cutting bricks. I don't have an angle grinder and don't really want to pile on the cost any more.
I found an online supplier of used firebricks which are a little cheaper, though not much! It seems delivery for any such materials is going to be £50 wherever you go. Prices are swings and roundabouts...some places have cheaper firebricks, but other bits and pieces are way more so it evens out.