Coat Charcoal Blocks with Boric Acid to Prevent Burning
Published at 20:09 on 16 May 2018
I bought a charcoal block to do some fusing of sterling silver on.
I’ve tried fusing metals on the firebrick that I normally use as a heat-resistant surface when soldering and annealing, and have found that sometimes the molten metal flows into the interstices between the grains in the brick and then, when solid, sticks to it. The result is virtually impossible to remove without some bits of firebrick material embedded in it. Moreover, the firebrick itself has acquired an undesired divot in its surface.
The charcoal block avoids that problem neatly, but it has a problem of its own: being charcoal, it catches fire. Even if I put the glowing/burning bits out with a wet finger, some of the surface has already combusted into carbon dioxide and literally vanished into thin air. A charcoal block’s surface thus quickly becomes unusably irregular.
Yesterday the idea came to me to coat one of the remaining good surfaces of my charcoal block with boric acid barrier flux and to set it alight. My theory was that since borates are used in fire retardants (and I believe in the antiafterglow compound that matches are treated with), the boric acid which would be deposited by doing the above was likely to prevent the carbon from catching fire.
It does! Plus, the porous surface of the charcoal absorbed a significant amount of flux, so the whole thing burned very prettily with a large green flame that lasted about a minute and put on quite the show.