Lead can become too hot without temperature regulation. At 900 deg., lead fumes is dangerous, and inhaled. I don't GUESS how hot my lead is. I keep a close check. Get a thermometer!
Uhmm... no. 900F is not a problem. Lead does not start to boil, that is phase change from liquid to vapor, till 1740C. Lots of misinformation on this. Do some basic research on materials properties.
https://material-properties.org/lea...melting-point-thermal-conductivity-expansion/
https://en.wikipedia.org/wiki/Lead
The real issues surrounding casting are basic safety. Don't drink, eat or smoke while doing it. Don't go touching your mouth, picking yer schnoz, or licking fingers. Cast in a well ventilated area to remove fumes from FLUXXING. Don't cast with your head over the pot. Keep all liquids away to ban the tinsel fairy. Wear appropriate clothing- jeans (no shorts, cutoffs or with holes!), long sleeve shirt helps (I wear a long sleeve T in warmer months and long sleeve flannel in winter), leather shoes (synthetics can melt if you spill a bit) and always wear at least crew length socks to cover the ankles. I know someone who was not wearing socks and a bit splashed on his ankle, but he was a ladle guy too. Bottom pour avoids that issue. After you finish, change your clothes and wash your hands thoroughly and before eating, etc.
As for lead contamination, I have been casting for years with no elevation in lead levels in my blood. I cast Civil War minies at 875-900F because a large bullet requires temps that high and a fast pour for a decent production rate and low reject number. I do know a couple folks who did test high for lead. Both are avid shooters with modern handguns that frequent an INDOOR range. A main ingredient of modern centerfire primers is lead styphnate. Couple burning that compound indoors with splattering bullets on a backstop just 25yds away and presto, lead issues.
With all that scary stuff out of the way, casting isn't hard. Just practice reasonable safety and make bullets. You'll probably find that 750F is a bit low to cast a heavy bullet and get a good product. Raise the temp a bit before adding any tin or other metal to your melt. Frosted bullets shoot just fine. Cold cast, misshapen, Quasimodo looking ones do not.