So, we know clearly that sodium silicate, a.k.a. water glass, can be used to join clay pottery pieces togeher during the clay modeling process. Then the craft is fired in a ceramic kiln and it all becomes permanent. But, what about joining ceramic together that has already been fired?
Oh yeah, right, you probably see the main motivation of this… repairing broken ceramic. Otherwise, it hardly makes sense anywhere else. Due to the stiffness of ceramic, if you want to join two arbitrary pieces together, you would need clay filler material to put in between.
The conventional wisdom from professional potters is that you cannot join back together a broken ceramic craft, put it back in the kiln, and make it whole again. Why is this? Ceramic undergoes permanent changes once it is put in the kiln. Moisture and permeability, the important aspects of clay that make it stick together and make it easy to stick other things to it, go away after the ceramic kiln firing process. Why does water glass work so well in the clay modeling process? Beyond being an excellent adhesive, the fact that it penetrates the permeable clay surfaces it sticks to also makes it tend to absorb into those surfaces to join the two into one. But, such cannot be the case with hardened ceramic, can it?
Well, let’s hold that thought for a while. First of all, I must state some distinctions from a theoretic standpoint. Firing ceramic in a kiln is not quite an analog for scrambling an egg. Carbon is a very chemically active and reactive element, able easily operate in a myriad number of different ways. Silicon, by contrast, is much more chemically inert. It takes a lot more effort to get interesting chemical properties out of silicon than it does for carbon, things that are less likely to arise at random in nature. Ceramic is significantly silicon-based due to the role that silicon-based clay minerals play in its construction.
So, a few points to tout about the theoretic feasibility of joining ceramic pieces.
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Is the ceramic structure pure crystalline, or is it amorphous? Amorphous substances can be quite feasible to join in a seamless way.
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Is the ceramic vitrified so as to be non-porous at the cracks? If it is not fully vitrified at the cracks, fusing the pieces back together can be feasible. Notably, ceramic dishware is typically vitrified so that it is impermeable to water even without an enamel.
Conventional and popular ceramic repair techniques often times come down to using carbon-based binders such as epoxy. This has the prime disadvantage that carbon is not as chemically compatible with the silicon-based ceramic, and that the various carbon-based binders will render the craft no longer to be food safe, it it was originally food safe.
20190401/DuckDuckGo water glass ceramics
20190401/DuckDuckGo water glass ceramic repair
20190401/DuckDuckGo sodium silicate ceramic repair
20190401/https://www.sheffield-pottery.com/SODIUM-SILICATE-WATER-GLASS-ONE-PINT-p/rmsodsilw.htm
20190401/http://www.lakesidepottery.com/Pages/Pottery-tips/How-to-fix-broken-pottery-china-ceramic-lesson-1.htm
20190401/http://www.lakesidepottery.com/HTML%20Text/Tips/pottery-magic-mud-magic-water-paper-clay.htm
20190401/https://en.wikipedia.org/wiki/Water_glass
20190401/https://en.wikipedia.org/wiki/Ceramic
20190401/https://en.wikipedia.org/wiki/Stoneware
20190401/https://en.wikipedia.org/wiki/Vitrification#In_ceramics