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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.

I'll plunk it in and see what it shows. If the EU calc shows excess silica (like I assume it would) then yes. edit: I'll use this recipe to play around with. (without any colourants)

Yup. @Suresh Sundaram, your video clip is showing mocha diffusion. If you go to the link @Hulk posted Hopper explains the process he is using. Quote below from it. Note that Hopper used a lowfire chrome red lead glaze for the red, chrome is well know to be volitive in the kiln. To recreate that look a cadmium inclusion stain could be used in a glaze or slip to get a similar colour instead of lead and chrome but probably wouldn't get the halos. "IN THE SOUTHWEST SERIES, THE DRY GREENWARE FORMS ARE SPRAYED WITH WHITE TERRA SIGILLATA, POLISHED WITH A SOFT CLOTH AND BRUSH DECORATED WITH A VARIETY OF BRUSHES USING BLACK-BRONZE PIGMENT. THIS IS THEN FIRED TO CONE 9 IN OXIDATION. THIS IS FOLLOWED BY PAINTING AND TRAILING WITH A CHROME RED LEAD GLAZE AND RE-FIRED TO CONE 010. ALL FIRING WITH THIS SERIES IS DONE IN AN ELECTRIC KILN IN OXIDATION. SOUTHWEST SERIES PIECES ARE PURELY DECORATIVE AND NOT FOR FUNCTIONAL USE."

@PeterH, using your train of thought... If a high calcium matte glaze recipe was put into the EU calculator then the calcium in the recipe reduced until the EU calculator showed no excess calcium then in theory it would be a gloss when slow cooled. I'll try plunking MC6G High Calcium Semi-matte one into the EU calc. and see what happens to the flux ratio. I'll try it with both wollastonite and another recipe with CaCO3 and see where the flux ratio lands. (might be tomorrow before I get to it)

@Christy Ann, like others have said underfiring a bisque isn't usually too big a deal but going forward if you use a "dirty" clay, like one of the dark brown bodies with lots of manganese or a red clay body, you might want to try bisque firing to cone 04. The extra bit of heat helps burn out more of the impurities as does doing a slower firing. (this helps avoid some glaze issues) It's hard to tell how a cone 6 glaze firing is going to go based on a bisque firing. Being thoughtful about how you load the pots does help. As a general rule of thumb electric kilns will fire cooler at the top and bottom and hotter in the middle. To avoid underfiring top and bottom areas load less mass in these areas, this means taller pots therefore less shelves. If you have shallow / short pots load them in the middle of the kiln using more shelves. Some people like to watch cones towards the end of the firing. To do this you must use proper eye protection every time you look in the kiln. (like these ones) If your target is cone 6 then you would place 3 cones in front of the spyhole(s), cone 5 (sentinel cone) cone 6 (target cone) cone 7 (guard cone). Once you see cone 5 tip down then expect cone 6 to be down in very approximatley 20 minutes. (depends on element life and kiln size). Having the guard cone will let you know if you overfired and by how much. It can be very awkward to see cones so not everybody likes doing this, above all be careful if you do. Just need a quick look, don't sit in front of the kiln and watch them fall. If your Amaco glazes have a recommended firing going to cone 6 then I would put the kiln on low for 2-3 hours, then medium for 2-3 hours then high until you reach cone 6. One thing that can make a difference in when the kiln sitter trips is how you place the cone in the sitter. If you find the kiln is underfiring (if only using the kiln sitter to shut off the kiln and not looking at the cone pack) then move the sitter cone over just a bit so the bar rests on the thinner side of the cone and the opposite for overfiring. Some people much prefer using sitter bars rather than cones so they are more consistent in how they bend and positioning doesn't matter. Like Jeff mentioned it's also possible your sitter needs adjustment. Easy to do if that's the case, video here showing how to do so if you need it.

What is your firing schedule for bisque? Also, are there more pinholes on trimmed areas? Once the pot is thrown do you rib the outside? Any chance you have some other clay handy that you can try the glaze on?

Hi and welcome to the forum. Have you tried the troubleshooting steps here? https://ceramicartsnetwork.org/faq or contacting customer service? (same link) If those haven’t worked for you let us know and we can forward this to admin.

Well that's a nice little bit of light bedtime reading isn't it? Glad I'm not the only one thinking precipitates effect calculated flux ratios. Interesting seeing the phase diagram overlaid with Stull but found it hard to decipher.

I updated the post in the pinned talc thread, the analysis for Fabi I posted is the same as the one from IMIFabi, company selling it. https://www.stobec.com/DATA/PRODUIT/3015~v~data.pdf There isn't going to be a huge difference in how @Bill Kielb s recipe for Marcia's Matte turns out using Fabi vs Amtalc. (I tried the recipe with Amtalc and found it very dry and unmelted on my claybody but hopefully it works for you) I ran it with Insight, swapped Amtalc for Fabi, just needed minor tweaking to get the formulas to match.

Wet or dry? If it's dry then nothing will have changed, if it's wet and a brushing glaze the organics to help with application will probably be shot. Can add some commercial brushing medium or make your own. Solubles in a dipping glaze could and probably have leached into the water in the bucket if it's still liquid. If this is the case then add a bit of saturated epsom salts solution to re flocculate it. Need to stir it all up and measure the specific gravity, if the glaze seems thin then add a few drops of epsom salts solution to a small amount of the glaze until it's the consistency you want then dip a test tile and go from there.

+1 Linda Arbuckle quote below regarding glaze calculation that I think pretty much sums it up for me: "The purpose of glaze calculation is to determine the total amount of each element present in a glaze, and the proportions relative to each other. With that information at hand, it is possible to calculate materials substitutions, revise melting points, and do other useful calculations. In the end, testing is the final proof, but unity molecular formula gives you a more informed method of choosing what to test." Are there qualifiers to interpreting the figures with glaze calc, yes of course there are. Glaze calc is another tool in the toolbox, one that can save a lot of work and material waste. I also use Limit / Target Charts, they aren't written in stone either but combined with UMF they can be a pretty good indicator if a glaze is worth mixing up and testing. An overload of boron, for example, might make a pretty and interactive glaze but I do question it's durability. Lots of lithium, I'm going to be wary of shivering. High level of sodium and / or potassium, odds of crazing are going to increase. Low alumina and / silica, glaze is not going to be durable regardless of flux ratio. And so forth. Recently there have been a few materials that have become unavailable to potters, Custer, Gerstley Borate, some talcs plus the EPK shortage. Looking at the UMF for any of these offers a way to find alternative materials and or a combination of materials that can be used in place of ones lost and bypass a lot of the trial and error testing.

Blue shades of stains don't use Cd (cadmium), cobalt is commonly found in blue stains (or vanadium for turquoise shades). For many yellows, oranges, pinks, reds and some purples a cadmium inclusion stain is used to supply the colour. More about Cd inclusion aka encapsulated stains here if you wish to learn more about them.

Hi @planter and welcome to the forum. @RiverOtter hasn't been on the forum since Sept 2022 so unlikely they will see your question. If you send them a PM they will get an email saying they have a new message. To send a PM click on @RiverOtters name or avatar then at the top of the new page that opens there will be a little envelope. Click that and follow the prompts.

And then use the pots. High boron in this glaze? Large amount of Gerstley Borate?

I went through the Techno File topics from Ceramics Monthly and the only one that struck me might possibly be it is the "Ingredients' Roles" article written by Steve Loucks in the Nov 2021 issue. It's behind a paywall but you can access 3 free articles a month. Basically Loucks looks at glaze components and lists them in a specific sequence of spars, fluxes, clays and glass formers then looking at the fired glaze in relation to what the ingredients do and what influence they have on the glaze. Alterations are then made to the base glaze to see how the different components change the base. There is a brief comment on UMF but not much. Possibly what you were referring to?

I'll try and do a search online for it in CM through ICAN, do you remember the author or anything else about it? Thing about flux ratios that doesn't make sense to me is using them to look at microcrystalline glazes. If a microcrystalline glaze is formed via the mechanism of the excess MgO, CaO, SrO, BaO, Al2O3, TiO2, ZnO, or MnO (or combination of these) that do not stay within the eutectic during the cooling phase of the glaze matrix then how does one measure for this? Same reason that COE figures don't work on semi or matte glazes.

@C.Banks, is this a snippet of the UMF article? I can't access the full article, if it is maybe somebody can find it? It's from William Carty , Matt Katz and John Gill. The Unity Molecular Formula Approach to Glaze Development March 2008 Ceramic Engineering and Science Proceedings 21(2):95 - 107 DOI:10.1002/9780470294611.ch10 In book: Materials & Equipment/Whitewares: Ceramic Engineering and Science Proceedings, Volume 21, Issue 2 (pp.95 - 107)

How so? I have started a new thread covering Stull, ratios etc here, might be best to reply in that thread to avoid this one going off topic.