Setting the Floor Thickness So You Don't Go Through the Bottom
Okay. You've opened your clay, you've got that little crater in the middle, and now you need to know how deep is deep enough without being too deep.
This is the floor. It's the bottom of whatever you're making, and it's the part beginners mess up in one of two ways. Either they leave it thick as a hockey puck and the piece feels like a brick when it's done, or they get greedy chasing thin walls and poke straight through the bottom with their fingers or a tool. I've done the second one. More on that in a second.
How thick is thick enough
For a beginner bowl or mug, you want somewhere between a half inch and five-eighths of an inch of floor. That's it. Not a quarter inch, not thin like a walls-of-the-piece thin. The floor stays noticeably thicker than the walls the whole way through, because it has to hold the weight of everything you're about to pull up around it, and because it's the part that gets trimmed later. If you trim a floor that started too thin, you'll go through it on the trimming wheel and just sit there looking at your hole. Ask me how I know.
How to actually measure it
Here's the plain method, no gadgets required at first.
- Stop the wheel, or slow it way down.
- Take a wooden rib or just your thumbnail and gently press down into the center of the opened clay.
- Feel for the bat underneath. Not "see" it, feel it. You want a slight resistance, like there's still real clay between your tool and the bat, not a mushy nothing.
- If you're not sure, there's a cheap trick: a wooden skewer or a needle tool poked straight down to the bat, then pinched at the clay surface with your fingernail, then pulled out and held next to your bowl wall. Now you can see exactly what a half inch of your clay actually looks like. This tool is like two bucks if you don't already have a needle tool lying around. Don't let anybody sell you a fancy depth gauge for this when you're starting out.
Do this check now, while the floor is still accessible and flat. Once you start pulling walls up, you're not coming back down here easily.
Why I check twice
I check the floor thickness before I open any further and then I check it again right before I start pulling walls, because clay moves while you work it. Sometimes opening the clay wider thins the floor more than you meant to. You think you left yourself a half inch and you actually left three-eighths, and you won't know until you're trimming a hole into your own bowl later. Two checks, thirty seconds each. Cheap insurance.
An opinion, stated as mine
Some people will tell you to eyeball the floor and trust your feel for it. I don't teach it that way, not for beginners. I'd rather you actually measure with a tool for your first twenty pots than "develop a feel" that turns out to be wrong twenty times. Feel comes later, once your hands have some data to work from. Right now your hands don't know anything yet. That's fine. That's why we're here.
The kid question
I had a Primary kid ask me once what I do all day. I said "I make bowls." She said "why." I didn't have a great answer, and I've thought about it more than I expected to since. But here's the closest thing I've got, and it applies directly to this lesson: you make bowls because a bowl that goes through the bottom isn't a bowl anymore, it's a ring, and rings don't hold soup. The floor is the whole reason the rest of the piece matters. Get it right and everything you build on top of it has a chance. Get it wrong and you're cutting the thing in half later just to see where you went wrong, which I have also done.
A caution that's real, not decorative
When you're checking floor depth with a needle tool, keep your other hand and fingers well clear of where the point is headed, especially once the wheel's moving again. It's a slow, controlled motion, not a jab, and there's no reason to rush it. Take it slow the first dozen times until your hand knows the distance without thinking about it.
At the end of the day it's a half inch of mud you're protecting, so don't overthink it into a science project. Just check it, twice, and move on.
Before next time: open and check the floor on three separate balls of clay, no walls yet, just opening and measuring, until pressing down and feeling that half inch stops being a guess.