Bubble crackle glaze6/13/2023 ![]() ![]() The reasons for this phenomenon are not because of gassing (this is demonstrated by the fact that high boron glazes often blister worse on a second firing). Boron can induce blistering, especially if its amount is quite high (check limit/target formulas for guidance).Does the recipe contain binders? When do these decompose to create gases (it might be higher than you think)?.Adjust the glaze so that it has a lower surface tension so that bubbles break more easily at the surface.Strontium carbonate can help smooth viscous zirconium glazes, small amounts of ZnO and Li2O can do miracles for glaze flow.more flux or a lower alumina:silica ratio). Reformulate the glaze to have more fluidity to heal imperfections (i.e.Or source them from a frit rather than raw materials. Reduce zircon or alumina in the glaze melt to give it better flow properties.Glossy glazes, however require extra attention. In fast fire, matte glazes automatically have this property because the formulations to make a crystalline matte and a late melting glaze are the largely same. If you are firing fast then you need to use a fast-fire glaze formulation so the glaze does not begin to melt until after body gassing is complete (the whole modern whiteware and tile industries are built on this principle).In fast fire we want glazes that remain unmelted until after 950C (gases from decomposition can occur up until this temperature) and then melt quickly after this. ![]() In slow fire we want glazes that are mobile and can heal imperfections over a long soaking period. ![]() The approaches to dealing with glaze chemistry issues differ in fast fire (e.g. Is the glaze recipe or chemistry the problem? Could you do a test on a small piece to confirm this? It might also work to adjust the firing schedule to soak, decrease the temperature a little (so the glaze is still pretty fluid), hold it and then cool quickly for the next few hundred degrees to solidify the glaze. Are you holding the top temperature long enough? Perhaps a much longer than expected soak might be necessary (on very thick tile or sculptural pieces, for example, 24 hours might be needed). You might be under estimating the amount of gases that are coming out. If you are using organic additives be aware that some of these can generate considerable gases during decomposition do tests without them, use an inorganic substitute or find way to disperse them better into the slurry. For example, use frits, supply CaO from wollastonite instead of whiting or dolomite, use cleaner clay materials, or use stains instead of metallic carbonates. Substitute these materials for others that melt cleanly. dolomite, whiting, manganese dioxide, clays, carbonate colorants, etc). Significant amounts of gases can be generated within the glaze itself due to the decomposition of some materials after melting has started (i.e. Generally a more fluid glaze will heal blisters much better (see section below on blisters occurring even after refire).Īre excessive gases generated during glaze fire? Often glazes appear like the melt should have plenty of mobility to heal but this can be deceptive, a melt flow testing regimen is the only way to know for sure (melt flow testers have a reservoir at the top of a steep incline and the glaze runs down a calibrated runway). 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. Blisters are evident on the fired glaze surface as a 'moonscape' of craters, some with sharp edges and others rounded. ![]()
0 Comments
Leave a Reply. |