Min

@Marilyn T, could you post the recipe, glaze specific gravity and which clay you are using? Also, does firing to 2185/2190F bring cone 6 down in your kiln? No soak at peak temp?

Another thing you can do to toughen up unfired glazed pots is spray them with laundry starch. (the aerosol kind, not the spritzer bottle)

I aim for 2 glaze loads a week. During the busy times I have 1 kiln firing while the other is cooling.

Hi Janette and welcome to the forum. There really isn’t a definite yes or no answer to your question. Some clear glazes can alter the colour of oxide colourants in unexpected ways. It depends on what is in the glaze composition. Example would be putting a clear glaze over a chrome wash or brushwork, if the clear glaze contains zinc the chrome will go brownish. Another example would be if you put a glaze high in calcium over an iron wash or brushwork, the calcium will bleach out the iron colour to lighter colour. You could also get unexpected running or blurring of the colouring oxides. There is also going to be an interaction between the white glaze and the clear. Like most things in ceramics it’s best to try it and see what happens. I’ld suggest making up some test pieces, see what happens with those before committing a “real” piece.

I don't think subbing RedArt for Lizella would be the best match. A bit like substituting an apple for a potato, they are both produce but not the same thing. Can you get Plainsman materials in where you live? Plaisman's Redstone might be worth testing if it's available to you and less expensive than Lizella. The people at Plainsman are really helpful, you could also email them and ask if they could make a suggestion. (their contact info at the bottom of the link below) Plainsman Redstone info here.

@Tammyt, I have one of those Cone Art 2327D kilns and I've never had it fire as quickly as it's scheduled. The more shelves/mass you have in it the more it slows down.

Hi and welcome to the forum. Zinc oxide will absorb moisture from the air if it's not kept in an airtight container and result in little hard lumps. You can re-calcine the zinc by firing it to 800-900F then regrind it or some people simply put the lumpy zinc in a blender and grind it up.

It's my understanding that copper carb releases carbon dioxide + water in the 300-320C range but then at 1050C it loses oxygen as it restructures (confirmed in this John Britt article). Since you are not firing as high as this it could continue to be an issue re bubbles. Re zinc, I think that if you are going to try it as an auxiliary flux I would be looking at using a frit that contains zinc rather than zinc oxide given that it is not an active flux at low temps below 1085C. (from Hamer's Dictionary of Materials and Techniques).

If the first glaze saturates a thin wall then you apply a second coat or glaze the clay can't absorb the extra moisture without pulling away from the clay. I'ld try adding some gum to the ash glaze and then apply the Leach glaze when the ash one is just dry enough to handle. Don't wait too long before applying the Leach glaze. BTW the David Leach II glaze looks like a cone 10 recipe that someone has added just a titch of boron frit to. Does't look like a cone 6 recipe.

If you do have the soy wax for container candles (the softer kind) you can add a bit of steric acid to it to firm it up. About 1 part steric acid to 10 parts soya wax. (It’s sold for hardening candle wax)

Looks like a cream breaking brown glaze. Sometimes called cream breaking rust or red or more recently there is a glaze called Folk Art Guild White that is the same type of glaze. High level of tin with a small amount of iron, application is crucial to get the breaking colour.

Ben, have you seen the work of Jeffery Nichols? He uses unglazed Velvet underglazes on the outside of his pots. Might be worth trying to contact him and asking if he has any research or further info on the durability of unglazed Velvets. Logically they are similar to a highly pigmented stained slip, just with less clay and more flux. https://ceramicartsnetwork.org/daily/article/The-Twenty-Year-Teapot-A-Potter-Discusses-the-Lengthy-Process-of-Developing-His-Voice-in-Pottery-and-Shares-His-Teapot-Making-Technique-Too Pots by Jeffery Nichols

There is a thread link below about Liquid Quartz with another link within that thread with more discussion on it. https://community.ceramicartsdaily.org/topic/30248-liquid-quartz/ What was the definition for archival from Amaco? Seems odd that they market their velvets for use with or without a covering glaze but could not say the velvet finish was archival.

Soy wax works great on the bottom of pots, also smells less than paraffin wax when burning off.

@ClayFishStudio, ask your local supplier if they can bring in some empty Amaco 1 pint glaze jars. I drilled the lid to fit the critter top, depth of the jar is a little deeper but other than that they work well. When I bought mine they were about $1- each.

Hi Paula and welcome to the forum. Glaze blisters are one of the harder things to figure out the cause of and remedy. Blisters can be from the glaze application, materials, the firing, the claybody, the application or a combination of these. Given when you refired to a cone lower it healed most of the blisters is a good sign. Do you use cones to verify your firing and confirmed you aren't overfiring? Have you tried an unfired piece at a cone lower or only a previously fired piece? This is where I would start trying to solve this. Since we don't have the recipe(s) for the problem glazes we can't offer suggestions for altering those but the claybody, firing and application method could be areas to look at. In this months CM there is a really good article on glaze blisters by Jeff Zamek, it is behind a paywall but you can access 3 free articles a month. It's well worth a read. Link to it here.

Oh gosh, sorry, don't know how I missed that. Okay, so plan B would be to make up a slip and use some oxides to make a black. Slip needs to go on greenware, as soft leatherhard as possible to avoid fit issues. Also test the covering glaze over it before committing a "real" pot. There are recipes for black slips that contain various amount of colouring oxides, most include iron + manganese + cobalt and sometimes chrome also. Something along the lines of this recipe from Vince Pitelka.

Hi and welcome to the forum. Have you tried black underglaze? It can go on greenware or bisque.

@davidh4976, good job on subbing the Custer out. I like how much silica and alumina are in the formula, will make for a good durable glaze. I could see a bit of iron in the recipe if you ever wanted to tone down the blue a little.

Are you soaking at the end of the firing? If so a 30 minute soak is going to add a lot of heatwork which could be making the clay brittle. Could you post a couple pictures, including the rims? When you say you could try a long soaking at the beginning what do you mean? edit: for sure it's not glaze shivering?

I had a look through Hamer's Pottery Dictionary of Materials and Techniques and under the Crazing topic. Snippet from there reads "Where a glaze is overfired and then crazes it is because the overfiring affected the body more than the glaze." also "The crystalline silica in the body becomes fused silica and in this form it has a much lower rate of contraction. The glaze still has the same rate of contraction and therefore in proportion it contracts more when overfired than when correctly fired." Hamer doesn't quantify how much overfired. I haven't seen it with a single cone higher firing, maybe more prevalent with very tight firing range claybodies?

I would verify with cones. You could fire the kiln with shelves (and some extra posts for mass) but without pots to calibrate firing with the cones. Is the design of the pieces part of the problem of chipping? Where are the pieces chipping, rims?

Thank you. I believe I made my point. If you would like to debate this further then please feel free to PM me, any of the other mods or admin.

With all due respect Bill I didn't say that. (however underfiring a clay and glaze will contribute to crazing) What I am trying to point out in the subtext is when someone asks a simple question we have the option of just answering the question or going into theory with much more detail and theory when oftentimes it really isn't necessary. When we have a better understanding on a person's knowledge and experience it is easier to tailor a reply to fit the question, if not perhaps we can offer a solution or answer with an offer to go more in depth or into theory if that is warranted.

I think we all know about clay and glaze COE's. After firing hundreds of glazes on thousands of pots in multiple kilns and types of kilns (wood, gas, electric) I've yet to see a glaze that fits a claybody then craze when fired a cone higher. Many of the kilns I've fired, mostly pre the advent of controllers, often had a variance of a cone from top to bottom. Underfiring created issues with the glaze at times, overfiring resulting in crazing didn't. Perhaps if you want to discuss theory of shrinkage, expansion, contraction, COE etc we could start a new thread.