Let's talk vitrification · Pottery People 💛☕️

Let's talk vitrification

Sorry that this is a bit long, but it's a detailed process that I find important. I'm sharing this in the hope that it helps others as much as it helped me. We've touched on this subject in a few posts lately, so I thought it might be useful to share this. There is more than one way to accomplish this test, as with all things. I'm not saying this way is better or other ways are wrong.

CAVEAT: I am no expert, and I didn't come up with this method, but am passing it along. My source is Tim See. Many of you know him. He has a Facebook group called Tim See's Beginner Throwing Group, linked here: https://www.facebook.com/groups/1835752663124936 where you will find references to this and many other subjects. It's a safe place to get accurate, tested information from a trusted source. Feel free to join his group, regardless of your skill level. It's a wealth of knowledge. Feel free to skip to the link at the very bottom for Tim See's instructions on making test bars.

Now on to vitrification. I find it important to know that my functional wares won't leak or absorb food residue. The only way I know to do this is to test the vitrification of the pieces. While I'm at it, I also test the shrinkage rates. It's amazing that there is sometimes a fair amount of variation from what the manufacturer posts. These variations can be from differences in firing schedules, raw materials, and more.

Here are some ground rules that I use religiously:

I always use cones on every shelf. It's the only way to verify the amount of heat work accurately.
I test every batch of clay, verified by the lot numbers on the boxes/bags
I keep a log of all my firings so I can refer back if there are any problems

To make the test bars, cut or form a bar from your clay. Mark it with a 10 CM line (it's easier to calculate shrinkage), and the wet weight. I mark both ounces and grams. Use an accurate scale like a beam scale. It matters!

Measure and weigh again at leather hard and bone dry, marking the bar each time.

Bisque to your normal bisque temp, weigh, measure, and mark.

Fire in your normal glaze firing but DO NOT GLAZE. Immediately upon removal from the glaze firing, weigh on an accurate scale and record the weight. Then soak in water for 24 hours. Next, boil for one hour, then remove from the boiling water. It will likely dry on its own, but if there are drops of water gently pat them dry. Weigh again.

You will also record the length of the 10cm mark you made. To determine the actual shrinkage, if you measured in inches use this formula: (original size - final size / original size) x 100. If you used centimeters, it's much easier. Convert to millimeters and subtract from 100. So if your 10cm line is now 8.8cm, or 88mm, 100 - 88 = 12, or 12% shrinkage.

As for your absorption, you will use a similar method. Subtract the dry, fully fired weight of the bar from the weight immediately after boiling. Divide the difference by the pre-boiled weight. Multiply by 100 to get your absorption rate. So using the top test bar in the image (Roo), the weight when removed from the kiln was 97.45 g. The weight after soaking and boiling was 97.70 g, a difference of 0.25 g. Divide 0.25 by 97.45, for 0.00256, times 100 equals a 0.26 absorption rate at cone 6. The shrinkage on that test was 13%. KMW gives a shrinkage of 11%, +/- 2%. Note that weighing on my digital scale gave a weight o 98 grams, while weight on my beam scale was 97.45. Accuracy here is important!

Here is the link to Tim See's full instructions, as well as a calculator where you can just plug in your numbers. Most importantly, there is a link at the bottom of this document to a shared spreadsheet where you can enter in your own results, and see what others have found with their tests.

https://docs.google.com/document/d/1h5Qd0XVzh__JK4uYKG5z6R98CTF7PFG9KUmmEaOd2u0/edit?tab=t.0

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