frosting

[peerlesscowboy]

-30* below zero here about daybreak this morning so I'm in no big hurry to go outside and feed the cattle 'til it warms up to maybe twenty below or so, thought I'd throw out a bone to chew on?
What actually causes frosting? I'm speculating that if air is in contact with the alloy surface when it solidifies that causes it to frost. Note the sprues always frost on top? When the surface of the bullets frost does this mean that they're shrinking enuf' inside the cavities before the surface solidifies to allow air to enter via the vents and come in contact with the surface? I've noted that if you get things way too hot, you'll eventually reach a point where you'll encounter noticable shrinkage along with heavy frosting! Does just a little frost mean you're getting just a little shrinkage?

John C. Saubak

[Bass Ackward]

Frost comes in two different types. That from being hot enough to cook out the tin near the surface of the slug. (Without some percentage of tin, you can not frost this way.)

Heavy frost that looks like granular salt comes from heat that is vented into the cavity from the adjacent cavity on an already solidified slug. Meaning that it can't draw in any new metal to fill again. So you notice it more on the parting lines. Kind of a gas cutting in the mold. In the old days of high tin mixes, the trick was to use no vent lines on the blocks at all and rely on top venting only.

You can either improve top venting, or add even more heat so that the slugs next to it don't solidify as fast and correct the damage, cast faster, or cut the tin if you are adding extra.

The first type of frost polishes off with a rag and reveals a nice shinny surface below. The second will look like the surface of the moon.

Bottom line is to get the heat right for the mold and then it doesn't happen.

[1Shirt]

Bass, an excellent analysis of frosting!
1Shirt!:coffeecom

[georgewxxx]

One thing that is usually not addressed about frosting, and that's the differences in boolit diameter. Like Bass stated, light frosting has little or no ill effects. Heavy frosting on the other hand usually has major shrinkage all around and sometimes one side of the mould is hotter that the other leaving the casting out of round and very noticeable during the sizing operation. You'll see gaps of untouched lead on the sides of the boolit. That will show up on the target if you compare lopsided rounds against normally ones. ...Geo

[Bret4207]

I have some moulds that seem to work best, that is they get the best fill out in my alloy, with a light and uniform frosting. I have been unable to see any size difference with them using mics reading in tenths. Now heavy frosting is another animal and is no good. BUT, sometimes I've had Lee moulds that required getting the mould up into heavy frosting temps and then cooling before they would work right. I don't know why, but using the frosting as a temp indicator sometimes, SOMETIMES, with a Lee mould that is otherwise just as clean and free of defects as any other, it seems to require getting it really hot for a few minutes to "fix" what ever the issue is.

As to what causes frosting, Bass's theory sounds as good as any other I've heard.