Borax Slate and How do you use it?

I just got home from helping a Art Teacher clean out an old cupboard.
In it was a square piece of - what I imagine is - slate. The box
was marked Borax Slate… It is about 5" square with a circular
indentation( not that deep but that wide ) about 4" in the middle.

Any ideas on what it is for and how it is used… Also we threw out
buckets of old casting sand that smelled horrible and asbestos
soldering pads, so that shows you how old this stuff was. I couldn’t
find it in the Rio Catalog, so hoped that some of you may have an
idea. I hate to throw out anything that MIGHT SOME DAY be

Thanks in advance.


Joan, The borax slate is used to crush borax and mix with water for
flux. That’s the traditional way and is still widely used, especially
in countries other than the U.S. Jerry in Kodiak where it’s just like
summer today, (rain, drizzzle and fog and 40 degrees)

I still use a borax cone and slate for most soldering operations -
but I’m rapidly turning to an industrial liquid fluxe as it tends not
to dispace the solder pallions as it heats up.

The slate is used to hold a small amount of water and a cone, made of
crystallised borax, is ground over it until there is a fine
suspension of the borax in the water. This is then applied to the
joint before placing the solder pallions and heating.

It works very well and can outperform liquid fluxes when you have to
heat up a piece for a long time. I prefer it when I am soldering large
silversmithing pieces.

Tony Konrath
Gold and Stone