What is the sand-like coating on power resistors?

[Unlisted]

Many of the power resistors have a coating that appears to be a
sand-like coating on them. In some ways, it looks like some sort of
concrete. It's too hard to be plaster, since it cant be scratched with a
fingernail, like plaster can.

I'm just curious what the material is?

 

[N_Cook]

On 15/02/2019 06:32, Unlisted wrote:

Many of the power resistors have a coating that appears to be a
sand-like coating on them. In some ways, it looks like some sort of
concrete. It's too hard to be plaster, since it cant be scratched with a
fingernail, like plaster can.

I'm just curious what the material is?

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

 

[pfjw@aol.com]

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

 

[N_Cook]

On 15/02/2019 13:57, pfjw@aol.com wrote:

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

I'm thinking of the usually white block type, a porcelain trough holds
the wire wound on a ceramic core, then sealed with something that is too
hard to be Portland cement, more like the hard glass of green or grey
vitreous coated ones. A dart point would not scratch either , but is
would scratch the coating of those old 60s/70s buff/yellow Portland
cement coated capacitors.

 

[pfjw@aol.com]

Both the white shell, and the (usually grey) infill are fired ceramics. Different composition gives different hardening temperatures.

Peter Wieck
Melrose Park, PA