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Min
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Posts posted by Min
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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?
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@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)
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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.
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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.
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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.
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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)
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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
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3 hours ago, CarolineL said:
Hello, I have a similar problem. I built a wood mold or frame and have the clay on top of it and it’s cracking even though keeping it wet. Is there something that can be put between the wooden mold and the clay like bubble wrap so it addresses the shrinkage problem?
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?
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Is the glaze made for brushing or dipping?
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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
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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.
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4 minutes ago, PeterH said:
I assume that you intend to reduce both calcium & silicon, or is the amount of silicon involved a negligible fraction of that in the glaze..
I'd wondered if there was a trustworthy limit formula you could use, be interesting to see what it says.
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)
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3 hours ago, neilestrick said:
Robin didn't do anything that fussy. It's probably just solubles creating halos.
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)
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@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.
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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?
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7 hours ago, BassClef said:
Hi, I joined at the Silver level but I can only watch previews in clayflicks. How do I fix this?
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.
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10 hours ago, C.Banks said:
I was looking for the trial version of the NIST? phase diagram software and this: https://phasediagram.weebly.com/eutectics.html reminded me of some language here.
I don't pretend to understand it all although I do grasp the larger 'chunks' and descriptions of parts getting squeezed out and reforming encouarged me to wonder how many of the principles might apply to the glazes we find most appealing.
There was a discussion of overlays for the stull chart that gets stuck because the diagrams are copyrighted iirc.
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.
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57 minutes ago, Suresh Sundaram said:
I did find an analysis on another post here but I’m not sure how accurate it is. In any case it’s different from Amtalc.
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.
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30 minutes ago, Elmoclayman said:
However, its been in this tub for about 20 years.
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.
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3 hours ago, PeterH said:
Hard to argue with that, but I would suggest that they can help reduce the number of tests you need to make.
For glazes that phase-separate (e.g. many mattes) the UML formula will not accurately reflect the composition of the different phases. Which raises issues of interpretation and limit setting.
+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.
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Just now, Judith B. said:
Thanks Kelly! So far, adding 10-20% ratio of satin matte glaze to test mixtures does indeed retard crystal formation...the more diluted the Dynasty glaze the less crystal formation. Cd inclusion stains sound like the way to go. I am getting a bit ahead of myself, but if they work out with Tangerine Ice and and I want to add them to the blue shades of Dynasty matte glazes I don't think they come in blues?
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.
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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.
A very simple cone 6 glossy base
in Clay and Glaze Chemistry
Posted
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.