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The tool that you have there is a sash mortice chisel probable 1/4", as its name suggests its for chopping mortices in sash windows.
Its length lets you cut deep pockets in sash windows.

You have a lot of work in front of you to knock it into shape JMHO but I wouldn't even think about using sand paper it needs to be ground.
You`ll have to be a bit brutal about this, if you have a slow turning grindstone great, if not then you will have to keep water handy so you don't take the temper out of the chisel.When you grind it don't try to grind a bevel on it,just have the back of the chisel up and the bevel side down feed the chisel into the stone at 90 degrees so you are just grinding steel away.

Keep grinding it till the back of the chisel has a decent face on it then you can start to grind the bevel on it the bevel needs to be about 35 degrees.
JMHO but the one thing a mortice chisel does not need is a hollow grind on the bevel,if it does have a hollow grind when used the chisel will bite into the wood and the edge will have no back up and as you use the chisel the edge will break off,this is what happens when bench chisels are used for chopping out mortices.

What is needed on the bevel IMHO is what is called a rounded bevel,that is you round the bevel from the cutting edge back into the chisel,this rounded bevel supports the sharp edge,that is it backs it up and the rounded bevel is used against the ends of the mortice to lever the waste out.

So if you have a grinder just give it a go,it will be good steel in the tool but if all I had was sand paper I would put it back in the bucket that's just the way I`am.Have fun Billy.
 

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The chisel is ground on its sides about 3/4" from the end - it tapers from 1/4" to a little over 1/8". I have a high speed grinder but 3/4 would still be quite a bit to grind off. I am thinking about cutting it off with a hacksaw to the point where it is full width, then try to reestablish the bevel according to the suggestions here. Are there any issues with cutting the length off vs grinding? I'm thinking it would be faster with less heat buildup.
 

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I know this is an old thread.. But finally got around to working on the mortise chisel. I got a Grizzly wet grinder a while back so I used it to grind the first 3/4" of the chisel off to get back to where it was full width, then I put on a 20 degree primary and 35 degree secondary bevel. It works pretty good now, but the bashed up handle made it hard to hold so I decided to try to make one. I've only turned a practice spindle before so I decided to make a test handle out of oak before making a final one out of ash or locust.. But the oak one came out nice so I think I'll just keep it. Just need to put on a coat of linseed oil and it will be time to chop some mortises.
 

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