1/13/2024 0 Comments Bubble le glaze![]() Be aware that this temperature could be lower than you think, be prepared for it to be 200 degrees F or more below the firing temperature. Failing that knowledge you can experiment with slow cooling the kiln through various temperature windows to discover the best results. Each glaze has a different point at which this happens and ideally, cooling the kiln to just below that temperature and soaking there is the best solution. ![]() This breaks the bubbles and provides enough time for the remaining melt fluidity to level out the surface. If that is not possible they need to be cooled to the point where the increasing viscosity of the melt is enough to overcome the surface tension that enables the bubbles to form and hold (of course that temperature is different for each glaze, it is normally found by decreasing the drop-and-hold temperature over a series of firings). Here is the key: Melt-fluid glazes need to avoid high surface tension. This can happen even with fine porcelain bodies. If the temperature is dropping rapidly they may not break at all, and if they do at some point, what remains of the melt may not have time heal itself. Such glazes that form bubbles can be soaked for hours at top temperature and the bubbles may not break. At the same time, such glazes often contain colorants and other materials, having high LOIs (producing a lot of gases of decomposition that become bubbles in the melt). 0.7 or more molar in a cone 6 glaze) so they melt well but at the same time having high surface tension. Reactive glazes are often of this type, those having high percentages of boron (e.g. How is it possible that a highly fluid melt can form blisters that do not heal? Bubbles can only form in a high-fluidity high-surface tension melt (analogous to the way bubbles form with soapy water). But very often, the opposite is the actual truth. I am about to tell you something that could fix a blistering problem that has nagged you for years! It seems logical that a glaze not healing its blisters does not have enough melt fluidity. ![]() Is the glaze too fluid? Is the surface tension too high? ![]() Generally, a fluid glaze will heal blisters much better, but only if it has time and if they have broken. Melt flow testers have a reservoir at the top of a steep incline, the glaze runs down a calibrated runway. A melt flow testing regimen is the only way to know for sure about how much your glaze flows and if flow is changing over time. Often glazes appear like the melt should have plenty of mobility to heal but this can be deceptive. Generally potters will encounter this problem much more than industrial producers (the latter use more balanced glazes and cleaner clay bodies). This problem is so serious that entire production lines can shut down when it hits. Blisters can vary in size and tend to be larger where the glaze is thicker. In some cases, there will be some unburst bubbles with a fragile 'dome' than can be broken. These craters are the remnants of bubbles that have burst during final approach to temperature or early stages of cooling. Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solutionīlisters are evident on the fired glaze surface as a 'moonscape' of craters, some with sharp edges and others rounded. Glaze Slurry is Difficult to Use or Settling Dunting and Cracking of Clay Bodies During Firing
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