Originally Posted by
geargnasher
"Frosting" is an effect caused by two things in combination: The presence of antimony in the mix, and a very hot mould.
The temperature of the metal going into the mould has zero to do with whether or not the boolit surfaces will have the "frosted" appearance, that is 100% a function of mould block temperature and cooling rate, or rather the SLOW cooling rate, of the alloy. The more slowly an antimonial alloy cools, the more time the antimony dendrite crystals have to form lattices, and the appearance of a textured surface becomes present.
For example, I can literally pour 800-degree wheel-weight metal into a 200-degree mould and get shiny, wrinkled boolits with rounded edges. I can also pour 650-degree wheel weight metal into a 450-degree mould and make boolits that are so frosted and undersized that they appear as rough and dull as freshly-broken cast iron, or appear as if they were sand blasted. The difference is the temperature of the MOULD, not the alloy. And, of course, the presence of some antimony in the mix. Tin/lead alloy or pure lead always casts fairly shiny, regardless of mould temperature or alloy temperature.
The general rule for most of the ternary (lead, antimony, tin) alloys that we scrounge up for making boolits is to run the pot below 750 degrees F., or really about 100-150 degrees hotter than the fully-liquid point, and to preheat the moulds somehow (dip a corner in the melt for a while, set across top of pot for a few minutes, or improvise a "mould oven" using a hot plate and metal box) so they come up to casting temperature faster or start out at casting temperature. THEN, as you cast, maintain a pace that is brisk enough to keep the mould hot enough for good fillout.
Casting good boolits is all about consistency. Constant pot temperature is important, this is what your thermostat and thermometer are for. Constant mould temperature is even more important, and that is controlled simply by adjusting the timing of the various pouring, waiting for sprue to set, cutting, opening, dumping, closing, and refilling operations. Casting boolits is like driving a car on the highway, you have to constantly make slight corrections to steering, throttle, brake, etc. and watch your mirrors, the road, anticipate hills and valleys to keep the speed consistent and in the "zone" that you want to be.
You'll have to experiment with timing operations and pouring stream adjustment, technique, sprue puddle size, etc. while watching for frosting, rounded edges, filled bases, air bubbles/voids in the bases, wrinkles, shiny spots, frosted bands/shiny noses, sprue flashover time to give yourself clues about what is working and not working with a particular mould, alloy, and atmospheric condition.
Here's a quick and dirty method that works every time. Turn on the pot and start melting your alloy. Stir it with a spoon as soon as it starts to get mushy and watch the thermometer. Once the metal thins out like thin porridge, keep a close eye on temperature, it should remain constant as the phase changes. Once the last bits of grainy metal go away, the temperature should start to rise again (the metal is past the phase platau). Record the phase plateau temperature and add, say, 150F to it. Allow the alloy to reach this temperature and adjust the pot to maintain it. Spoon in a layer of pine sawdust on top of the melt and stir the alloy gently to flux and reduce oxides. Skim if you want, or not. Dip a corner of your mould down int he metal and hold it there until the alloy no longer sticks to the blocks when you withdraw it, this could take anywhere from 30 seconds to two minutes. Dip the tip of the sprue plate in the alloy too, for about ten seconds, then close the mould and start casting. The mould should suck the pot temperature down 50 or so degrees depending on the size of the mould. Fill, cut, and dump the first few castings quickly and glance at the boolits. If shiny and wrinkled, keep casting culls as fast as you can until they start to fill out and get an even, satin frost all over. From this point on, you can play with the timing and sprue puddle size to control mould block and sprue plate temperature to keep the mould temperature and boolit quality even from nose to tip.
This works with any alloy, any pot, any pouring technique, and any mould (except non-antimony alloys won't frost, you just look for sharp edges and good fillout with those) and any weather. Don't forget to glance at the pot temperature once in a while and make sure it stays 100-150 above the fully-liquid point that you recorded first thing. It's all up to mould temperature and technique after that, and trust me, quit dinking around with pot temperature, it is not going to do you any good.
One more thing, to emphasize what MT Gianni wrote, IF your thermometer is off even 200 degrees, if you do as I described above, it won't matter because you only want to obtain an alloy temperature that is a certain amount above fully liquidus with ANY alloy, and your thermometer is good enough to give you that valuable reference point even if it is not giving you an exactly accurate temperature reading.
Gear