Min

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.

As we all know both clay and glazes shrink as they are fired and then cooled. A cone 5 clay fired 1 cone hotter will probably shrink a small amount more than if fired to cone 5 or lower. An extreme example would be to compare the size of a mid or high fired piece of bisque to the same piece when fired to fired mid or high fire. It shrinks. We also know that both claybodies and glazes mature over a range, there isn't one magic temperature that either is mature. Given that crazing is most often a result of a claybody being too "large" for the glaze which results in the glaze "stretching", ie crazing, to the body as both cool (or X amount of time later in regards to delayed crazing or from moisture absorption). By firing a midrange body one cone hotter will likely shrink it ever so slightly from what it would be one cone cooler. It won't make the body larger therefore chance of crazing is reduced. Grossly overfiring a body will increase the porosity of the body but that's another issue. As to if the glaze will have issues with firing 1 cone hotter, perhaps it will, and like all things ceramic it should be tested. edit for clarification: referring to the physical claybody shrinkage as it relates to crazing. @Bill Kielb, would you mind clarifying what you mean by the glaze and "tensile failure" in regards to the op's question?

Yes it can over time. If you have 3134 then you could reformulate the formula with that which would give you room for more epk in the recipe. Or add a small amount of epsom salts solution to help flocculate it. (after measuring specific gravity)

Yes. If you want a more extreme test then go from a 300F oven to ice water. Boiling to ice water has a temperature difference of approx 180F whereas oven to ice water is approx 268F difference. I used to do a sequence of 300F/ice water then 310F/ice water then 315F/ice water but I think that once you have done 1 cycle of oven/ice water you have stressed the glaze to a point where further cycles aren't valid.

I like throwing that clay but it made my skin itchy. Have you found this? I like how nice and tight it is, have some mugs and bowls made from it, parts unglazed and no staining after many years of everyday use, slip made from same body without sand brushed on unglazed areas.

Hi Zuri and welcome to the forum. I looked up the Lavafleck and see that it has a firing range of 1200 C - 1270C. Clay is at its strongest when fired to maturity so if possible I would suggest raising the 1240C that you currently fire at. (and adjusting glazes as necessary)

I use 4 main glaze colours for dipping glazes, all made from one base plus a clear liner, these are in 5 or 6 gallon buckets plus an accent glaze that is just a small container of glaze. When I used to spray glazes I had more glazes but could mix up smaller amounts of them since I didn't have to have a big bucket to dip them in. I find that using less colours makes a more cohesive looking display, I don't try and please everyone by offering every colour under the sun.

That's how I feel too and probably why I have stuck with ceramics for so long. So much too learn and experiment with, one lifetime is barely enough time to scratch the surface. I am especially interested in the chem side of ceramics because I always want to know the why of things or what happens if ... Ever since I can remember I have had my hands in something. Started with textiles (batik, weaving, sewing) then wood (furniture) then clay. Thanks for posting my question Pres.

My suggestion for a QOTW would be what other creative things are you involved with? Thanks to @Hyn Patty for sparking this question with her post here where she said "But I also do a million other things too. Sculpting, mold making, resin editions, oil painting, etc and not just ceramics."

Thanks Tom and Bill, I do have diamond Dremel bits and diamond pads plus the green silicon carbide grinding wheel so I'm sure one way or the other I can get it sharpened.

Trimming tool I mentioned above arrived yesterday. Edges were hardly sharpened at all. I gave it a try this morning, not great. I'll try sharpening it but definitely will not be buying another.

Appreciate the people who have shared their recipes for a clear glaze. Getting a well fitting clear is one of the harder glazes to come up with. For those new to mixing their own glazes, be sure to test the glaze for FIT before making up a big bucket. Glaze fit to a claybody is no different than fitting a pair of jeans. What fits my body won't necessarily fit everyone else's body. If you use a high expansion glaze on one side of a pot and a low expansion one on the other there is a good chance of the pot dunting / cracking. If someone is looking for a clear glaze to use with or over stains or underglazes or some specific colouring combinations (like chrome tin pinks, reds, purples etc) then you need a specific type of clear glaze that is hospitable to the colourants.

A for effort Pres! Appreciate that it must be hard coming up with a new question every week.

Hi @Triciachapman and welcome to the forum. Just to confirm, it's the same make of slabroller and just the bottom roller isn't turning. If you slowly hand spin the bottom roller you will see 2 small set screws near both ends of the roller. Are they still there or is one or both missing?

@ChristopherW, I would not expect the recipe you posted to make a durable glaze. (but it will be a very expensive glaze to mix up given the price of Gerstley Borate now)

Agreed it's best to test small amounts before trying to alter an entire 5 gallons of glaze. Adding silica works up to a point in glossing up a glaze but if the amount of silica needed exceeds the amount of fluxes that are present to dissolve the extra silica then you reach a point where excess silica can't be taken into the melt. Worth a try but might need a plan B if it doesn't, this would be adding more flux along with the extra silica.

Hi JLR and welcome to the forum. Going forward you might want to consider applying the underglaze on greenware instead of bisque. I understand you wanting to save this load but not only is it tricky underglazing vitrified clay you are also going to have problems glazing the pots as glaze doesn't like sticking to vitrified clay either. Sometimes we just have to move forward and start again.

For a large piece I would roll out a slab of the same clay as your sculpture is made from, make it about 1/4 - 3/8" thick then cut strips of it about 3/4" wide. Dry the strips between boards (or drywall pieces or whatever). When you go to fire your sculpture lay the strips down first, gaps between them, and sit your sculpture on them. (don't need to bisque fire them but make sure they are bone dry before firing and fire slowly) What you are trying to do is raise the piece off the shelf and allow air circulation underneath the piece to help even out the temperature. Re slowing down between 1000F - 1100F both when heating and cooling, it's at approx 1060F that quartz inversion happens. As the quartz crystals change from an alpha crystal structure to a beta one there is a volume change of 1 - 2%, this can stress the clay and cause cracking / dunting.

Hi and welcome to the forum. I would look at what the slow glaze program is going at during the 1000F - 1100F range, if it's more than 50F / hour then I would slow it down during that ramp. I would also slow the kiln down during cooling during the same range and not open the kiln until it's at room temp. This is something I would be looking at doing to help equalize the temp within the kiln and doing it slowing as that is a large piece and there will be substantial differences between the 1.5" thick areas, where it sits on the shelf and thinner areas. If you aren't sure what your glazes will do in a slow cool then I would suggest running some test pieces through the firing before your large sculptural piece. I don't know what your final cone is that you are firing to but another option would be to fire it to your target cone using the slow bisque program instead of the slow glaze program then add a slow cool. Look up your make of kiln and what the preprogrammed firing programs are and compare the two. (I'm going to delete your duplicate post in the Chemistry section)

Or you could use Pieter Mostert's Brongniart calculator. You need to know the weight of the glaze materials within the glaze slurry. Brongniart's formula works this out, either longhand or with the calculator. https://pietermostert.github.io/SG_calc/html/brongniart.html

Just to confirm I'm reading this right, the clay is cracking as it's shrinking on the wood mold/frame? You are keeping the clay wet?

Is the glaze made for brushing or dipping?

I bought one of these from Bailey quite a few years back, can't remember if it had a blue haze on it when I bought it but it definitely rusts. They are calling it a Scraper Rib, it's steel, but not stainless and not flexible. https://www.baileypottery.com/c-128-048.html

When I enter the MC6G High Calcium Matte 2 into the EU calculator I'm not seeing excess to the eutectics of CaO even though this is coming in at 0.90 in the UMF. I'm going to assume this glaze uses a different matting mechanism, perhaps the excess alumina and silica causing the matting or a micro rippled surface. If that's the case we need another recipe to test the theory with or reduce the silica and alumina perhaps.