Calibrating your clay and your printer

Measure your own clay's shrinkage with a shrinkage tile and your printer's case fits with a case-fit coupon; your next molds are made with what you measured.

A data sheet gives a clay's typical shrinkage, and Castworthy cuts its printed cases to fits that work on most printers. Your slip, your kiln and your printer are your own, so measure them once and your molds follow. Everything you measure is kept under Calibration on your account, with the day you measured it and the pieces that use it.

Your clay's shrinkage: the shrinkage tile

Make a shrinkage tile: choose your clay, how it goes into the mold (slip casting or pressing), the cone you fire to and, if you like, a name for your firing schedule. You get a small piece: a test bar 140 mm long whose mold marks two lines exactly 100 mm apart.

1. Make its mold, print it and pour the plaster, as for any piece.
2. Cast or press three bars or more. A bar cast in slip is cast solid: keep it topped up level with the mold's back as it sinks, and the bar's pour sheet says how much slip to have ready. Measure between the bottoms of the two lines when each bar comes out of the mold, and scratch its number on its back.
3. Measure again at leather-hard if you want to know how much is drying. Dry the bars slowly and flat, and fire them as you fire your pieces.
4. Measure each bar fired and enter the readings: on the bar's pour sheet, or under Calibration.

Each bar's shrinkage is the lines fired against the lines when made, wet to fired, as data sheets give it. Your figure is their mean, with their spread (from two bars on) and the data sheet's figure beside it.

A new piece of that clay, in the tool for the same process and at the same cone, takes your measured figure first. The Size tool shows it with its spread, and what that spread means for the fired size; you can still choose the data sheet's figure or type your own. The pour sheet says which figure the mold was sized with, and so does the recipe in its files.

A piece sized before you measured keeps its size until you size it again: its sheet says when your figure has moved.

Your printer's fits: the case-fit coupon

Print a case-fit coupon for your printer and the filament you print cases in. It is eight small parts, cut by the same code as every case:

• two halves that close on a tongue in a groove: the seam fit;
• the two clamps that slide over their flanges: the clamp grip (how hard they pull the seam shut) and the clamp clearance (whether they catch on the flanges' edges);
• two cups for a spoonful of plaster each, one making a plaster key and the other its socket: the natch fit of plaster keys;
• two printed natches, one's nipple into the other's socket: the printed natch fit.

Try each one and say whether it was tight, good or loose. Tight loosens that fit by 0.05 mm, loose tightens it as much, and good keeps it. The next molds for that printer and filament are cut to your answers; a fit you set by hand on a piece stays that piece's. Print another coupon to check: it is made at your new fits.

Your printer's usable bed

If your printer prints less than its catalog size (a clip, a wiper or a purge line takes some of it), measure what it really prints and keep it under Usable bed. Your pieces on that printer are then checked against it and their print files split to fit it.

Read next

• Reading the pour sheet: What the pour sheet tells you: the plaster and water for each piece, the slip to have ready, what you need, and the steps with their times.
• Printing the cases: How the print files are laid out for your printer, what to print them in, and how the clamps, edge frames and printed natches go together.
• Making a slip-cast mold: From a 3D model to a plaster mold you can print and pour: bring the model, set its fired size, pick the pieces, generate, and get the print files.

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