thx but not load bearing. checked first,

Circular saw with diamond blade to cut the plaster layer. It will make LOTS of dust, so be ready to deal with that.

Make sure you determine where any wiring and plumbing runs are in the wall before cutting into it. Also, will both ends of the half wall be supported by a connection to other walls and/or to cabinetry? If not, you could end up with a wobbly half wall.

I agree that a diamond saw (or any decent fine-toothed blade) will cut the plaster without any problem. However, I've been very cautious with cutting plaster so that I don't end up with a lot of patching afterwards. I've had very good luck with a Dremel multi-tool (the one with the end cutting blades) to cut out areas for receptacles, but that may be too tedious for an entire wall.

Plaster is quite hard to penetrate, and you will get unpredictable fracturing around the cut line. I would use wood, a long straight edge, or at least a double layer of painters tape to stabilize the surface, and then cut through the plaster with gradually deeper cuts. The first cut would score and penetrate the hard skim coat. Subsequent passes would gradually cut the rest of the layers of plaster.

Is there wood lathe behind the plaster, or is it rock lathe (which looks like sheetrock)? If it has wood lathe I might consider first cutting the wall about 6-8" above the final cut line, and then using deep clamps to hold boards on both sides to stabilize the wood lathe as its being cut. Vibration breaks the plaster because it is so inflexible, and you want to keep the wood from moving as you cut.

Bruce

As was asked above, how will you stabilize the half-wall without a post or cabinets? Cabinetry can help if the boxes can be screwed to wood cleats fastened to the floor.

This job will generate incredible amounts of dust. It may be easier and sounder practice to remove the whole wall and build a new reinforced half wall from scratch.

Whatever you decide to do with demolishing the wall (all or partial), also be prepared for the plaster disposal. Plaster is very heavy, and in my area you have to dispose of it like you would dispose of cement and other construction materials. I cannot put it out for standard garbage pickup, so i had to take it to a commercial disposal site that charges by the pound. It quickly filled up my pickup truck bed, and I had so much that I almost exceeded the weight capacity of the truck.

When one piece of plaster (2x3 ft?) suddenly came loose from the ceiling and fell while being demo'd it was so heavy that it snapped a wooden window sill right off the window when it fell.

Bruce

"We checked" that it wasn't load bearing. Who is we? A structural engineer with a written report? Because that is what you need. Houses old enough to have plaster walls didn't use always modern building techniques. A house that old needs more than a neighbor who has done a renovation to guess as to it's structural nature.

And it also needs someone who knows how to deal with lead and asbestos remediation. Becaus you will definitely have the first, and may have the second. If you have old ducting in that will, then I'd have to say you definitely have the second as well.

This isn't easy or cheap to do correctly. It can have far reaching consequences to the structural integrity of your home as well as your family's health.

Use a Sawzall with a wood blade long enough to cut through both sides and the framing simultaneously. Maybe tack a wood straightedge on each side for guidance, depending on your skill level. Much less dust than any grinder or saw and much faster. Put a dust mask on yourself and the person holding the shop vac.

Cut an inch and a half lower than the height you want, because you're going to have to nail a new sill to connect the ends of the framing.

Of course before beginning, confirm the bearing status of the wall and any mechanicals buried inside.