Why Does Borax Eat Firebrick?

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So I've been looking into it a little, and I haven't found an exact answer to this. It obviously does eat it away, and too the looks of it in my forge, sort of melts it. I haven't found the exact reason it eats it like this.

I was hoping someone here with some chemistry knowledge, or that has found the answer over time can share. My best guess is something to do with the high heats, and borax being caustic. While looking it up, I found that some refractories use boric acid as a binder actually, when they are needed to be chemically resistant.

 

[Stacy E. Apelt - Bladesmith]

This may not be 100% excat, but tyhe basics are:
Borax becomes boric acid when heated It will eat the calcium/aluminum/silicon-oxide (CaAl2Si2O8) out of the blocks leaving the less strong glass/silica compounds. Additionally, as the liquid soaks into the firebrick it expands greatly and pulverizes the now more fragile brick, leaving holes in the forge floor.
Coating the floor with something that is hot acid resistant is the cure. Bubble alumina, Casto-lite 30, Mizou, and similar refractories are the best choices. A piece of kiln shelf can also be used to protect the floor.

 

[Atlas Knife Company]

The 94% alumina products have much higher resistance to borax and other acids. Plistix 900f, Greenpatch 94, basically turn to hard ceramic and nearly immune to flux. Much better resistance than Kastolite LI-30 or Mizou. I can't explain why aluminum is dissolved and alumina is nearly immune.

 

[Greenberg Woods]

Molten glass is INCREDIBLY corrosive and will destroy nearly anything it is in contact with, and seeing as borax is often added to glass and behaves like one, id be willing to bet the process is similar.

The Corrosive Nature of Molten Glass

One of the outstanding characteristics of molten glass is its ability to chemically and physically attack the refractories which serve to contain the glass. The physical attack usually consists of penetration of voids, cracks and joints by the molten glass or in some...

link.springer.com

This paper with a free abstract implies that a variety of processes are going on. One of them simply being that alumina is soluble in molten borax and can simply be dissolved.

I think it is less of caustic reaction in the classic acid/ base model as there is no solution for H+ ions to build up, and i think its more caustic in the sense of pouring hot water on a sugar sculpture.

High temp chemistry is some wild stuff, and this paper seems to note that a mix of processes are at work to break down anything in contact with molten glass/ borax.

 

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Molten glass is INCREDIBLY corrosive and will destroy nearly anything it is in contact with, and seeing as borax is often added to glass and behaves like one, id be willing to bet the process is similar.

The Corrosive Nature of Molten Glass

One of the outstanding characteristics of molten glass is its ability to chemically and physically attack the refractories which serve to contain the glass. The physical attack usually consists of penetration of voids, cracks and joints by the molten glass or in some...

link.springer.com

This paper with a free abstract implies that a variety of processes are going on. One of them simply being that alumina is soluble in molten borax and can simply be dissolved.

I think it is less of caustic reaction in the classic acid/ base model as there is no solution for H+ ions to build up, and i think its more caustic in the sense of pouring hot water on a sugar sculpture.

High temp chemistry is some wild stuff, and this paper seems to note that a mix of processes are at work to break down anything in contact with molten glass/ borax.

Very interesting. This is exactly what I was hoping to learn.