Potters & Sculptors - Making Rock from Mud
It seems that many if not most glaze recipes have an addition of 2% bentonite. This is usually explained as being added to suspend the glaze better in the bucket, not to make a better finished glaze. I think some people have lost sight of the reason for adding it, and just automatically add it to any glaze they make up.
I would think that any glaze that already contains 10% or more kaolin or ball clay would not benefit from adding bentonite. Just wondering if anyone has additional knowledge on the subject.
I found this really interesting, as I also have some recipes that call for addition, and some that do not...
I agree with you George in that if a glaze already contains clay, i am hesitant to add Bentonite. I have a jar of pre-mixed Bentonite and water now that I add to a glaze as I get annoyed of that fact that it is hard to mix in. I believe and like that helps in making a glaze a bit harder for handling reasons prior to firing.
The use of bentonite is to hold a glaze in suspension when the glaze is short on other ingredients/ clays that would do the same job. It is the best clay for doing this with the least amount used. So it will have the least chance to change the glaze in any way. If your glaze stays in suspension you have no need for adding bentonite. Bentonite will absorb up to 10 times it's own weight in water.
This subject came up at a work shop I took with Hesselberth and Roy .They seem to feel that it did no harm and was more often a benefit so why not.
Very easy to dry mix from the start you need to per-mix it with water if your adding it to a mixed glaze.
I've noticed glazes with large percentages of Nepheline Syenite specify Bentonite in addition to 10% or more kaolin, clay, or gerstley borate. As Digitalfire says, the salts in Nepheline Syenite are slightly soluble and they become more so over time - which is not such a problem with feldspars. As Brian Guffey says, wood ash (or Soda Ash, or Pearl Ash) create the same problem, and excess bentonite can help suspend large ingredients like "color crystals" for a time.
If I exclude the bentonite from Nepheline or Wood Ash recipes I quickly get a glaze hard-panned on the bottom, even with the addition of Calcium Chloride which is better for the task than Epsom Salts. Adding the calcium chloride makes dissolving any bentonite much easier as well.
(In his "Glazes Cone 6" book Michael Bailey suggests Calcium Chloride can react with bone ash to create a hard deposit on the bottom of the glaze container, but I have not found this to be the case. I assume this must be a reaction of the small amount of Magnesium Phosphate in bone ash with Calcium Chloride to precipitate additional Calcium Phosphate, the primary ingredient of Bone Ash.)
Many glazes with a high level of frits and very little or no clay benefit from bentonite both for suspension and pre-firing glaze hardness. The problem is bentonite radically changes the look of some high percentage frit glazes (those more than 50% frit). Bentonite can make translucent frit glazes opaque and shiny red lead-cadmium frit glaze (CM-941) into a dull lava black leechable mess. In these glazes I use a gum with preservative, such as Xanthan Gum and bleach, to suspend the glaze - at least for a while after remixing the glaze.
Other glazes I've made which add bentonite to a glaze already containing 10% or more of kaolin, clay or gerstley borate, become a thick pudding, like a pint of Amaco glaze.
This is one such recipe, which say it must be applied very thick to fire Emerald.
But without the bentonite it seems to work just as well. I've seen some recipes calling for as much as 8% bentonite and I disregard this.
I initially learned much about adjusting glazes from this Pete Pinnell article.
I too would love to hear other ideas about bentonite / veegum / macloid. Particularly about other glazes whose fired finish are significantly altered by bentonite.
Jello pudding vrs. Coffee & cream That was the difference for me. Wish I had read this earlier. Used a recipe with high clay and bentonite it was like Jello pudding. wouldn't dry on bisque, toooo thick, wouldnt dry, added more water,still pudding. Found another recipe that left out the bentonite, and I had perfect coffee & cream. Made test tiles of both batches and will fire next week.
Recipe was Slatin's gnu blue. his had the bentonite.
Revised from J Britt, no bentonite.
75% Alberta slip 15 Gerstley Borate Dolomite 5 Silica 4 Rutile 4
I used 1/2 calcined slip & 1/2 un-calcined .
Now what do I do with the Jello Pudding????
Bentonite and feldspar mixed with clay makes incredible slab glue - it's really unbelievably better than slip.
I typically add more of this mix to the clay I'm using than the recipe calls for and whip it up with a stick blender. It acts like contact cement. Put the glue on two pieces of leather-hard clay, and once you touch them together they're very difficult to pull apart.
|Pat Horsley’s Score No More|
|Join pieces of leather hard clay together, handles or other attachments.|
|1,000%||Clay - the type you're using|
|20%||Feldspar Custer Potash|
|20%||Gum Arabic / Xanthan Gum|
I have a bucket of that Emerald pudding glaze Norm was talking about and am about ready to trash it. I've tried adding a little sodium silicate but was afraid to overdo it. Any ideas on how this glaze might be salvaged?
I tamed our quart of emerald pudding with Darvan.
If your alternative is throwing out your emerald pudding glaze why not risk over-deflocculation with more sodium silicate and sodium carbonate?
The good part: The silicate or carbonate ions both bind with the soluble flocculating cations magnesium++ and calcium++ (usually present as sulfates but also added by the bentonite) making them unavailable as insoluble silicates and carbonates. These are replaced by deflocculating sodium cations. Both also deflocculate slowly over time by making the glaze pH more basic, creating deflocculating alkaline lignosulfonates.
The bad part: Adding too many sodium+ cations will hardpan the glaze by neutralizing the repellant charges between the clay particles, but much of this excess sodium can be thrown out with the clarified water once the glaze has settled. I've been able to reflocculate any hard-panned glaze with calcium chloride.
The reason I switched our studio to Darvan was because the excess sodium in our deflocculated waste-water was killing the plants in our garden.
Thank you Norm for clearing my head, I will take a quart of the Emerald and work on that. I actually went to the link you provided and read the article on sodium effects on clay flocculation but as usual with the flock/de-flock conversation my head was spinning by the time I finished. There were a few Ah-Ha moments though.
It's an old story, the Emerald was very beautiful in the 1000 gr test batch so I went ahead and mixed up 6000gr. I don't have darvan so will try the silicate, we have a rock driveway so that will be a great place to dump the decanted sodium water.