I put on the concrete cladding last weekend and can't wait to fire up for the first time. How long should I wait before starting a few small curing fires? How long should I leave the concrete to set? I was thinking 10 days? should I wait longer?
I put my final render coat on mine then 2 hours later I was eating pizza , I know it was a bit soon but i dint get any cracks at all , oh and Iv had it going 4 times since and still no cracks . But I did have propane burner in after every layer for a few hours
to clarify, I haven't any insulation on yet let alone any finishing (render or other covering) done. I have the basic firebricks covered in 4" of concrete. I know heat will escape if i fire it with no insulation but I wasn't planning on baking or cooking for long periods, I just want to try out a pizza. I don't want to destroy it thought by starting out too early. What's the general rule of thumb for waiting before the first fire?
Not sure how long cec left his before fully firing, but my advice is to go as slow and as long as you can with your curing fires.
You can start your curing fires straight away....first try some scrunched up newspapers....2 or three times a day, then move on to kindling....again keep in small and frequent.....after 3 or 4 days try small fires with thin logs......keep it small for the first week. Gradually build up the intensity of the fires for another 10 days (if you've got the patience)
We call them curing fires, but in reality all you are doing is slowly driving the moisture out of your oven.(there's a lot of moisture in an oven).....the mortar/concrete will set in a couple of days but the curing process of any cement based product doesn't start until the 7th (ish) day. The general theme is that if you heat up masonry or concrete which has water in it, the water will expand and try to find a weakness to escape......normally resulting in a crack.
Normally the builds on here(not all) follow the method of fire-brick, then thermal blanket, then vermiculite concrete. You say you have put concrete directly on the fire-brick.....don't panic. This is the method used by the great Allen Scott when building large vault/barell ovens mainly used for baking large batches of bread. He used refractory concrete on the fire-bricks then laid aluminium foil on the concrete. These large builds were usually in a housing. He then poured loose vermiculite in the void....with no thermal blanket (from the AS builds I have seen)...These ovens take a long time to heat up but they hold the massed heat very well allowing the baker to do multiple batches on a single firing.
You still have the option of putting on a thermal blanket, but as you put the concrete directly on the bricks you may find that a vermiculite concrete mix may now be the best option.
Apologies for rabbiting on a bit.
Let us know how you get on......and if in doubt ...keep asking.
Yes it is an Alan Scott oven that I am building (although I have changed a few things), I got a CD from Rado on www.traditionaloven.com/. I actually pinged a mail to Rado last night too and he agreed that a 7-10 day wait was about right. The risk is that heating too fast may cause cracks but also cooling too fast because I have no insulation on yet will cause outside to cool faster than inside and that can also cause cracks. So bottom Line is I might try a few very small fires after 10 days and then get on with the insulation before I fire it up fully.
Here's an extract from the info from Rado (hope he doesn't mind me posting)
"DRYING FIRES: After the cladding was finished on the top wait 8-10 days to cure, 7 days in warm summer. Lots of water went in, also chemical water is present (which gets removed gradually by higher temperature.) Then the drying fires, start with smaller not too small fires but not too intense either. About 15 minutes for the first ones then 20 and 30 minutes long. What this does, it not only removes water but also firebricks adjust by movement with the cladding. Repeated mild fires expand it just little and then shrink again with cooling. If you for instance fired up high on the first time the hot face firebricks will will push on the cladding too much. And secondly, the water gets removed; you will see water drops around the flue area, these will collect mainly on the exit, many of them. It is the condensed water which get gradually removed completely. So no steam would build in the mass, steam is powerful (remember the banging wet rocks around cam fires?). You will be fine with this. You can set two fires a day if you had the time, normally 5-6 fires do fine. Just soak the heat through the dense part. The heat soaks through the top cladding in one hour or 3-4 hours after firing/cooking through the floor.
It would be wise if the oven was insulated, if not permanent insulation was done then at least if it's temporarily covered at least with some blankets, any heat insulation helps. It's quite important. It gets hot on outside of the cooking/firing part. Inner layers get hot fast and expand, while outer layer is still cold. This happens anyway (the hot face inside adjusts with cladding) but temperature differences in the dense material even up quicker when the oven is insulated. If not, especially on the cooling down after firing/cooking, the outer layers start to cool down (shrink) faster and cracks could developed on the outside. However the oven would still be functional after as per normal. You get also less heat climbing up and minimal heat disappearing out to the open air with a good insulation. Plus slower cooling down, it is the same as with rising up temperature but in reverse. Every time when you cook in future and especially if it's colder outside, close the oven door so it also cools down gradually."