petrichor Posted December 27, 2020 Report Share Posted December 27, 2020 Hello, I recently mixed up a "shino" glaze: n.syenite, spodumene, soda ash, ball clay. This is, if I remember correctly, an "American shino". It seems to be working fine: I like the satin-matte texture and how it reacts with iron and creates yellow freckles. However! The glaze also seems to have some sort of crackling going on. It looks like it is solid and not going anywhere, but there are tiny visible crackles, like the ones you get with crackling glazes. I cannot feel these with my finger. Does anyone have an idea of why the crackling happens? Is this something that can happen to most glazes if you mix up an unbalanced mixture? Does it mean that it is not food safe, hygienic, stable to use? Any tips and thought is very much welcome. Happy new years! Link to comment Share on other sites More sharing options...
Bill Kielb Posted December 27, 2020 Report Share Posted December 27, 2020 Tough subject. Shinos are not necessarily known for their durability and are often applied thick so they will crack to which the user intends to carbon trap for effect. Generally a glaze that crawls or is crazed is not considered to be a dependable liner or even durable glaze. It’s hard to know durability and solubility without some pretty purposed testing of the clay / glaze / firing combination so many folks use a liner glaze where food will contact and the shino as decorative. While we do not have a specific thing or things to know if a glaze is balanced we do know that durability is related to flux ratio and glazes that depart significantly from an R2O of 0.2 to 0.3 are less likely to be durable. But in the end testing is the only way to know, with erosion from the dishwasher often being the worst. Oh to add, glazes crackle and crawl due to composition and drying. If the components of the glaze shrink a bunch and / or the fired surface tension is such that it will not heal during a firing then they crackle and crawl. Shino users often apply their glaze heavily to ensure it crackles as different Shinos have a different propensity for this. A couple examples below of how folks often use shino and its characteristics to decorate. One Shino artist went for all decoration, and one uses a liner glaze on the inside of the ware with a very thin application of shino on the outside. Link to comment Share on other sites More sharing options...
Hulk Posted December 27, 2020 Report Share Posted December 27, 2020 Hi Petrichor, There are several crazing glaze themed threads on this site; try searching craze, crazed, crazing and read on! If you haven't yet found Tony Hansen's website, it's a real treasure, imo (as is this forum, thanks to all the folks who post here); here's his glossary entry on crazing: Crazing (digitalfire.com) From there, also see his many several articles on evolving low coe glaze recipes, reformulations, comparison of oxides, glaze chemistry and melting behaviours, oxide interactions, etc., etc. You might post the recipe you used, closeup pics of the crazing, and identify the clay as well - for more specific reactions from forum contributors... Link to comment Share on other sites More sharing options...
Min Posted December 27, 2020 Report Share Posted December 27, 2020 4 hours ago, petrichor said: Does anyone have an idea of why the crackling happens? Is this something that can happen to most glazes if you mix up an unbalanced mixture? Does it mean that it is not food safe, hygienic, stable to use? Like Bill has said there are two main issues you bring up, crazing and durability. I don't use the term "food safe" as I have never seen a scientific definition of what this actually is. Crazing is okay if the claybody is vitrified and good kitchen hygiene is practiced, the crazing does weaken the pot though, whether it's enough to be relevant is up for debate. If clay and glaze are both fired to maturity then crazing is often the result of the glaze and clay not fitting each other. In effect the glaze is too small for the pot so as it cools in the kiln it shrinks more than the clay and has to relieve the tension by cracking apart resulting in the craze lines. Other issue is the durability of these type of Shino glazes. Like Tom has suggested you could post the glaze recipe but chances are it doesn't fall within the suggestions from "limit charts". Glazes need to have certain minimum and maximum amounts of alumina and silica to make them durable and not have an excess of some of the other oxides. https://digitalfire.com/article/limit+formulas+and+target+formulas Link to comment Share on other sites More sharing options...
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