Glazes need to be gelled, have thixotropy. That means these things won't bob up and down to find the right place. The one on the right is completely useless, the scale is too wide. Most glazes need to be between 1.4 and 1.5 specific gravity (40-50 on this scale). That is like reading seconds on the hour-hand of a clock. And it is too long, when are you going to have enough glaze to float that? And the small one? It has a scale of 1.2 to 1.45. But it is a real bobber, it needs a fluid that gives it the ability to move. That will never be the case with a glaze. It is much better to weigh a measured volume of glaze slurry and calculate the SG instead. The easiest is a 100cc graduated cylinder, if 100 ccs weighs 140 grams, that is 1.4 specific gravity.
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