A pure stone formed long ago in ocean beds from shale and clay, slate is essentially compacted mud. It isn’t as hard as igneous rocks, like granite, and isn’t hard to cut with a grinder. It chips easily, however, so care is necessary.
To cut slate, you want a diamond-tipped cutting wheel for your grinder. Because slate is layered, it’s hard to draw a precise straight line onto it. This task gets easier, and you’re going to create a more accurate cut with fewer edge chips, if you first lay masking tape and then score the line into the tape with a knife. Cutting the slate from the underside is another way to minimize chipping on the good side. Neither measure is necessary in case a certain amount of inaccuracy and chipping is acceptable.
Making the Cut
As hardy materials go, slate isn’t very hard, and you can usually cut via a 1-inch or thinner slab in one pass with a 4-inch angle grinder without the requirement to keep the blade moist. If you’re cutting a slab that is thicker than 1 inch, score a line with just one light pass of a 10-inch grinder, then use the score line to steer the grinder since you cut through the slab on the following pass. It’s important to wear a dust mask when cutting slate.