On Friday, February 15, 2019 at 3:44:39 AM UTC-5, N_Cook wrote:
> My guess its like epoxy potting , where epoxy is pretty expensive so
> spun out with mixing with chalk .
> In the power R case, fire-cement , padded out with sand
It is, in fact, sanded grout - which is a mix of white Portland cement, sand and water, often with an admixture that permits very thin coatings. As the material cures, it is then passed through the sand to get the coating. Look up SikaFlex or Thoroseal to understand the concept. But there was (and is) no epoxy involved - not that you stated such. So, it would be a bit of phenolic board with a nichrome wire wrap, crimped ends, coated in grout, then rolled in sand to oversimplify the process.
Keep in mind that "sand resistors", while not precision resistors by any description are rather delicate beasts and acted as a sort of fusible link in many radio designs from the 40s through the 60s - most especially those that were AC/DC/Battery.
Peter Wieck
Melrose Park, PA