Getting the Right Volume

I've been creating cups, and now mugs, for Regulus Coffee House.  Although the process has been really fun, it's been a challenge to get all the pieces to hold a specified accurate amount of volume.  He told me the tasting cups he wanted should hold between two and two and a half ounces of coffee, and the espresso cups between five and six ounces.  My first thought was, "wow, that's not a lot of room for error!"   As anyone with an appreciation for handmade items knows, it's difficult to get all the pieces to be the same, much less to have just a half ounce difference!

So what have I learned from creating two dozen tasting cups, three dozen espresso cups and four dozen latte mugs (and counting)?  Well, there's the obvious... everything has to be measured.  Each piece starts with the same amount of clay, so I've done a lot of weighing.  And the project goes much faster if all the clay is de-aired, weighed, and wedged, before the throwing every starts. 

Then the not so obvious...  how exactly do you figure out the height and diameter of a cup that has to hold a specific amount of volume?   For this, I have to give what may be an unpopular answer to some... with math.  I had to look up the formula for a cylinder, and with the help of a spreadsheet, I figured out the proper height and diameter, remembering to add a 1/4 inch to the height so the coffee had some room, and that the height and width ratio was really up to my aesthetic preferences.  I used a Lid Master to figure out what the height and diameter of the vessel should be before shrinkage, and finally I had my measurements!

Throwing gauge

But it's hard to throw them all the same!  And measuring every one with the Lid Master was too time consuming. So I made a makeshift dragonfly, a small throwing gauge which originated from Japan.  Yes, I could have probably bought a really nice throwing gauge, but my need was immediate and my pocketbook light, so I made one using two bamboo skewers from my kitchen and a hot glue gun. I used the short, top part of the dragon fly to measure how deep I opened, the width of its "wingspan" to measure the opening of the cylinder, and I used the long, bottom part of the dragon fly to measure the depth of the cup.  It worked like a charm and cup production went quickly.  The last point to note however, was that it was VERY critical that the floor of the cup and its wall met and a hard right angle.  If not, I could lose up to an ounce in volume.  Who knew?!