Kingfish, I seriously doubt that you are heating your metal to a point that you would drive tin out of the melt. Tin and lead make a true solution, if they didn't when the melt cooled we sould have two different batches of metal. Tin oxidizes out of the melt as a result of contact with oxygen, but once the surface is coated with this tin oxide, no further tin oxidizes out unless the surface is disturbed, exposing new metal to the atmosphere. 10 pounds of Linotype contains about 6.4 ounces of tin, it would take a lot of surface exposure to oxidize that much tin.
For a deeper, more insightful conversation about this, contact Bill Ferguson at
www.theantimonyman.com Bill is a metalurgist with who runs a business selling folks like us antimony and tin. He has a vast knowledge about lead and lead alloys.
I use the Lee Production Pot, bottom pour. I make ingots using the little cornbread irons. These are about 3 1/2" long and about 3/4" across. They drop into my pot and melt very quickly and allow me to make quick additons to my pot as I am casting. By keeping the pot topped up, I almost never have to flux the pot itself. Some people use a layer of kitty litter on the melt to create a boundry between the melt and the atmosphere, but I've found that once that first layer of tin oxide forms, dropping these ingots into the melt exposes so little new metal to the atmosphere that any tin loss is completely negligable.
I know this goes against the conventional wisdom of the past, but I assure you, give it a try, you will be surprised. If you use a dipper things become problematic as each dip exposes new metal to the atmosphere. Good luck, enjoy the Linotype.