Firescale on commercial castings

[Aurarius]

I've been practising stone setting on some of Cookson's cast silver rings. The last two I've done were not resized before setting as they were the size I wanted them to be already. In other words the rings were never heated by me with any attendant risk of firescale forming. However, I noticed that once the stones were set and I was doing the final polishing considerable amounts of firescale became apparent around the shanks.

My question is, is it normal for commercial castings to show significant amounts of firescale like this? I thought oxgen was supposed to be excluded during the cooling process to prevent firescale forming.

[Dennis]

On the face of it no, it shouldn't happen Mark, but only Cooksons themselves can really answer. If they don't come on here, I should phone Birmingham. Dennis.

[Aurarius]

Many thanks, Dennis.
I'm not too troubled for the moment by the presence of firescale on these rings as they're mainly just practice pieces, but it would be an inconvenience if every silver casting I ever worked on was like this. Most of the stuff I fabricate myself now gets a coat of Boric acid in meths, and firescale rarely happens; to find that big manufacturers can't eliminate it from their castings is a bit disconcerting.
Mark.

[caroleallen]

I get a lot of castings done and quite a few of them have firescale. I don't know why either but I always make sure everything gets well sanded down before polishing.