That brick is a normal commercially produced house brick!
The heat was generated in my Adobe oven.
Quote an old post of mine;
I built my Adobe Oven several years ago and it is functioning beautifully.
I used a different recipe than the normal Adobe mixture and found it to be waterproof and had no problems with break down due to long and high heat firings. (compressed air assisted).
(1) 2 Units of Calcined Gypsum. (C G)
(2) 2 Units of Portland Cement.
(3) 4 Units of Clay / Sand Mix.
(1a) C G = Plaster of Paris; To make your own; Your Nursery or Hardware store will have Gypsum in bag form, the stuff used to break up clayey soils. To make a test sample take an old large frying pan 3/4 fill it with Gypsum straight out of the bag and heat over stove ( gas or electric ) till the color has turned to white by driving out the moisture and gas bubbles have ceased to break the surface. At this stage notice how the Gypsum has taken on a state that can only be called "liquified powder". If you bought "coarse" Gypsum, after calcining it place it in a concrete mixer with 10 pieces of broken house brick, cover the mouth of the mixer with plastic secured with string to stop the fine dust escaping. Engage the mixer for at least 2 hrs ( the longer the better ) this will pulverise the C G to a fine powder. ( the finer the better and stronger )
For larger amounts I place a SS sink over an open fire and "cook" the Gypsum.
(2a) Portland Cement; Straight out of the bag.
(3a) Clay / Sand Mix; = 15% Clay / 85% medium to fine clean Sand rundown.
Clay must be CLEAN; e.g. free of dirt or fossils.
I also pulverised the Clay/Sand mix in the mixer and run it through a 3/16 sieve.
Assuming you have a Plug or shape formed up (internal oven shape).
Mix the above recipe "DRY" and 3/4 fill a wheelbarrow. At the shallow end of the barrow add about 1 gallon of clean water and mix in enough dry mixture to form a "stiff" workable compound. Trial and error will tell you when you have the right consistency.
Do not mix up to much as the C G will go off before you can use it. The other advantages of this method are you can stop and start operations when ever you like and it's clean to work with (no MUD).
Apply 1 hand full at a time with firm pressure and tab or batter into position making sure an over lap occurs with each hand full this helps the bonding. When restarting on old work lightly mist the previous work to help adhaesion. Remember too much "WATER" will cause shrink cracks in new work.
To fix shrinkage cracks simply fill the cracks with C G/Clay powder, (50/50) and lightly mist with water several times this allows the deeper cracks to be penetrated with moisture.
Theoretically this mixture will not shrink as normal Clay or Portland Cement dose when water is added for one basic reason, Calcined Gypsum "EXPANDS" when setting thus equaling out the shrinkage in the Clay and Portland Cement.
There is no need for a roof however a coat of Bondcrete and a 1inch layer of Stucco over the finished oven will guarantee protection from the elements.
With this method you will see it as fun not "work".