Cutting clay accurately matters more than it sounds, because an even slab or an evenly divided block saves work at every later stage. The tools are simple: a wire, a harp with a fixed height, and guides.
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The wire
A length of wire with toggles at each end cuts a bag of clay into slabs, takes a pot off the wheel and divides a block into weighed lumps. Twisted wire leaves a textured cut some potters use decoratively, and plain wire leaves a smooth one. Wires stretch and eventually break and are cheap consumables rather than permanent tools.
Harps and even slabs
A clay harp is a wire in a frame with notches at set heights, so each pass takes a slab of the same thickness off a block. For slab building that evenness is the whole job: uneven slabs dry unevenly and crack at the joins. It is the cheap alternative to a slab roller and it does the same thing more slowly.
Guides for rolling
Two battens of equal thickness either side of the clay, with a rolling pin run across them, gives a slab of exactly that thickness without any special equipment. It is the oldest method and it works perfectly. Where a studio uses one thickness constantly, a permanent pair of guides on the bench is worth making.
Weighing rather than guessing
Cutting a block into equal lumps by eye gives pots that vary in size, which matters as soon as you want six of something. Cut roughly, then weigh each lump and adjust. A cheap kitchen scale used this way is the difference between a set and a collection.
Questions people ask about clay cutter
What do I cut clay with?
A wire with toggles for blocks and for taking pots off the wheel, and a harp or guides for even slabs.
How do I get slabs the same thickness?
A harp with notched heights, or two battens either side of the clay with a rolling pin run across them.
Should I weigh clay for repeat pots?
Yes. Cutting by eye gives pots that vary; weighing each lump is what makes a matching set possible.