Milling out ingots is one of the most deceptively complicated things that we do. There’s a lot going on at once in the metal. Your metal shows signs of being pushed too hard, too fast though the mill or being substantially overheated during the pour. Also, I’m not a fan of open faced ingot molds. They tend to make bumpy and irregular ingots that require extra effort to mill out. That’s another possibility of what is leading to all of your problems. I prefer round or square two-piece ingot molds like one of these.
It’s also possible that it’s a combination factors. Like I said it’s complicated. It could even be hindered by your torch adjustment. You said are using an oxygen-acetelyne torch. You could be using an oxidizing flame (too much oxygen), instead of neutral flame. A flame with too much oxygen on molten silver will tend to make the metal be brittle.
This is a common thread here on Orchid. Here’s a similar question someone asked in June. Maybe that will be of some help.
I wish I could tell you exactly what to fix. I suppose if it was me, I’d get a two-piece ingot mold first. They come in a variety of prices and sizes.
Keep us posted!!
Jeff