So I have this small post vise. It is very rusty and I was taking it apart to wire brush it and clean it up a little. I was hammering on a small wedge-shaped piece in the mounting bracket, and blip! this little piece of metal broke off and went zinging into my finger! (The finger is fine now) I had previously also accidentally broken off a corner of the mounting bracket, a year or two before. The vise is obviously forged, not cast (I can see the forge welds) but how/why would it be so brittle?

You talk about the vise and the mounting bracket as if they were the same thing.  Why? Mounting brackets are often discarded and remade and can be very different from how the vise is made.  I believe the Columbian mounting brackets were cast in some instances.  Also a smith could pick up a piece of high carbon steel to make a repair piece bracket and it could be overly hard.  If so draw temper on it. (especially during the Great Depression; I've run across a lot of "whatever was to hand" repairs from that era!)

Well, I can see a forge weld in the mounting bracket too. But perhaps the little wedge piece is high carbon steel. I just hope the rest of the vise won't send shrapnel my way when I am hammering on it...

I have a Columbian and an Indian Chief, both have cast mounting brackets. I'd be willing to bet donuts for morning coffee the wedge is what's at hand steel. The new wedge in my 4" Columbian is leaf spring come to think of it.

Frosty The Lucky.

The wedge in my vise looks like it was a horse shoe in a former life. In fact the heel of the shoe is very much untouched. (grin) It was available at the time and has served me well for many years. I will forging a proper looking wedge when this one wears out. (Bigger Grin)

Thanks for clearing up my questions guys!