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Min
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Posts posted by Min
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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.
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7 hours ago, C.Banks said:
I don't remember much about it other than gave me reasons to avoid decyphering the UMF.
I aquire magazines and some years I read less than others so this goes back at least three years and probably closer to five.
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?
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On 1/9/2024 at 9:44 AM, C.Banks said:
The one I'm thinking of is from an old ceramics monthly from the ? 90's?
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.
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@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
- In book: Materials & Equipment/Whitewares: Ceramic Engineering and Science Proceedings, Volume 21, Issue 2 (pp.95 - 107)
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3 hours ago, C.Banks said:
There is old criticsm of certain aspects of glaze chemstyr as being overly complicated and a bit elitist - the UMF system in particular.
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.
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Hi Chloe and welcome to the forum.
Are you in Surrey England? Maybe add that to your location if you are so it avoids confusion going forward.
Which claybody are you using and have you asked your local supplier if they can recommend a dry commercial clear glaze that fits it?
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20 oz mug, 1 lb (base isn't as narrow as it looks in the picture)
- Hulk, Kelly in AK, Pres and 2 others
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5
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Starting a new thread to discuss Stull, Flux Ratios, Silica:Alumina Ratios and the like that came up in this thread as it started to go off on a tangent into these areas.
All comments, theories and hypothesis welcome.
Do you look at Stull or ratios or limit charts or none of these? How useful do you find glaze calc software if you you it?Stull chart, do you see where your glaze(s) are on it and do you find it helpful in predicting a glazes attributes or altering a glaze?
Flux ratios, do you try and stick within 0.3 R20:0.7 RO ? Do you use this ratio for high calcium, barium, strontium, or magnesium matte glazes?
Silica to alumina ratios, do you use these to predict or alter gloss levels?
Boron levels, have you found “high” boron levels have an effect on glaze durability? Have you found glazes really high in boron are prone to crazing even though COE figures decrease with increasing amounts of boron?
Do you choose not to use glaze chem calculators and rely on line blending or a similar type testing or a shotgun approach testing and altering a glaze?
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Printers blanket, top side is the cream colour and has a very fine texture, much less than the canvas usually used for slab rollers. Bottom side is blue rubber material. Another advantage of using these is they don't bunch up like can happen with canvas sometimes when starting to put it through the rollers.
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Claybodies should be vitrified enough to not weep / leak even with no glaze on them when used for functional work and fired to mid or highfire.
Is this the clay you used? https://tuckers-pottery-supplies-inc.shoplightspeed.com/mid-smooth-stone-speck-clay-cone-6-stoneware-ecom.html
If that is the clay then in theory the clay should be vitrified enough to not leak with a posted absorption figure of 1.0 % but published figures can vary by +/- 1%. I would suggest running your own absorption tests. This is a good idea to to when using a new clay or every once in a while on an existing clay as materials can change.
How to on testing for absorption here if you need it, about 2/3 the way down the page, I would suggest doing the weight calculation test with several samples placed in different areas of the kiln.
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3 hours ago, craftykrb said:
How do you know if the clay is fully vitrified and that the glaze fits?
Link here on how to determine if your clay is suitable for outdoor work that undergoes freeze and thaw cycles. https://community.ceramicartsdaily.org/topic/23472-frostproof/
Link here on one method of glaze fit testing. https://digitalfire.com/test/bwiw#:~:text=-Select samples (or shards),ice water for three minutes.
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21 minutes ago, Dick White said:
In my studies of glazes, it is the high boron in this one when layered with other lower boron (conventional theoretic levels) glazes. Higher boron levels create a borosilicate glass. Lower boron levels create an alumino-silica glass (not quite soda-lime bottle glass, but the chemistry is moving in that direction). Layered borosilicate glass and alumino-silica glass do not mix to a homogeneous blend, but rather they are immiscible and flow through each other in rivulets or create a faux-oilspot effect.
Good example of how layering 2 dissimilar glazes can often result in very interesting glaze effects.
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Katz himself might have thrown a wrench in the works in regards to his testing into flux ratios, also into further exploration into silica:alumina flux ratios. From his 2016 NCECA presentation taken from this pdf.
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If in doubt pay the (approx) $35- and send it to get it lab tested. In this case we are talking mainly about the boron so choose that as the oxide to test for.
As to how much boron, I would be looking at drinking water guidelines. One here, an oversupply of boron in drinking water can have a nasty side effect, especially for males.
Stull Charts, Flux Ratios, Silica:Alumina Ratios - Open Discussion
in Clay and Glaze Chemistry
Posted
@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)