You may not need to "add tin" if you get the mold hot enough. Get the pot up to temperature and scoop the crud off the top. If ladle casting it must be "in there" and full temp, thermometer monitoring. Now, put the corner of the mold in the melt and wait for a little heat-up. Now, lube the mold and pins. Put the corner back into the melt. When it starts smoking, wait 20 seconds or so (amount depending on results, all molds are different), then start casting. Is it pouring evenly or is it coming out clumpy? If so, you can add 8" or so of your solder. No need to mix. It spreads evenly through the pot instantaneously. That's all the tin you will need. Also, 50/50 works best. Otherwise, the noses crack and are brittle.
