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: 1. I always use cones on every shelf. It's the only way to verify the amount of heat work accurately. 2. I test every batch of clay, verified by the lot numbers on the boxes/bags 3. 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.