Effect of Bentonite in glaze slurry

Walter Ivan Heath

12 novembre, 07:52

I’m not sure if it makes any difference, but I see some people adding 2% bentonite to my JP Purple .25 recipe (and A2/A2V based glazes)…more often than not, people do this to recipes that don’t have bentonite…it usually doesnt make much difference, but it is chemically the same as adding 2% more, in this case, EPK…bentonite, physically, improves the dry strength of the raw glaze, and, in some cases, is the only clay component, for example Val’s Turquoise…being the only clay component gives us the ability to flocculate the mix and have clay particles to floc…I don’t add bentonite to most of my glazes unless they either don’t have enough clay component to floc or the dried glaze is powdery…I always use unmeasured pre-wet CMC in my mixing water, don’t weigh it because it will all burn out anyway…my point is added bentonite can chemically influence fired outcome, for instance, the first time I weighed out Val’s Turquoise, I noticed it had no clay component other than bentonite, and I remember thinking “Oh! That’s why it is so copper blue…smart guy that Val Cushing!”…in many copper glazes, the higher the clay component of clay, the greater the color shifts away from blue and towards green…so we should keep in mind, something as routinely adding bentonite to glaze recipes that don’t have it, is slightly altering the chemistry, maybe not adversely, but still effecting chemistry…in this case with a chrome/tin glaze, we have the possibility of a color shift caused by 2% bentonite, not necessarily adversely, but still a possibility.
Please keep in mind, there nothing wrong with adding bentonite, just be aware of the usually subtle changes it can make in the fired results