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Making a proper fire brick or refractory liner Answered

If you are cheap like me then paying an arm and a leg for commercial grade refractory mixes or ready made bricks is no option.
And how wants to make their design only to find there is no matching bricks for it...

There is lots of totorials online and here on Instructables that deal with making heat resistent bricks and similar.
And this is all well and good for normal melting applications or your pizza oven, not so much if you really need intense heat.
I found that commercail mixes tend to be either really brittle once heated up or that they will glace up and even melt.
So why not make my own mix...

Fine sand
Crushed Perlite
Sodium Silicate

The first three you find in basically every home depot or garden center.
The last can be made from crystal cat litter and sodium hydroxide.
Use proper PPE please!
For a refractory mix I use:
100ml of clean water.
60g of crystal cat litter.
About 35-40g of Sodium Hydroxide.
This will provide a very silica rich solution.
Use a high jar or similar and fill the water in.
If you have use a magnetic stirrer, otherwise be prepared for manual labour.
Add some cat litter but not so much that stirring becomes a problem.
It will fizz around a bit and when it stops slowly add some sodium hydroxide.
The mix will heat up and the cat litter starts to dissolve, once it is gone add some more.
If it becomes hard to dissolve them add a bit more sodium hydroide but avoid letting it get too hot and so it boils!!
A bit of steam is fine though.
Towards the end you should have a quite hot mix with all cat litter inside and a bit of sodium hydroxide left.
See what dissolves and only add as much as you really need.
40g is enough, if it won't dissolve the cat litter then keep stirring every now and then and add some external heat, like placing it in a water bath on your stove.
Once all is dissolved the mix is ready and should keep the jar closed to prevent it turning it a rock hard cement...

Make sure your dry ingredients are well mixed and relatively fine in particle size.
Take a defined amount of your mix, like 100g and see how LITTLE water in ml is required to turn in into a plyable mass like green sand.
Note down this relation to get the right mix with the waterglass.
To keep the mix workable you want to add the same amount of clean water to the amount of waterglass you calculated.
If it is quite cold day you can leave it undiluted but give it a test on how long it takes to set on the surface to get your working time.
Make sure you really mix it all properly to enuse the waterglass wets the surface of all aggregates in the mix.
Press into your prefered form, remove form and let air dry, preferably on some wooden sticks so they won't accidentally fuse with the surface you have put them on.
Once you have enough to fill your kitchen oven put them in at a heat of about 80-90°C for 3 to 4 hours.
Open the door every now and then to let the moisture escape.
After that they can be fired up to become fully fire resistent.
Do this slowly as there might still moisture be trapped inside that needs to steam off, not boil off as it might crack the bricks.

Once they got a good glacing from being used they might fuse together but should not constantly crack and melt like some commercial products - plus it come dirt cheap in comparison.