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Min
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Posts posted by Min
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1 hour ago, neilestrick said:
I have found the Speedball underglazes are more stable at cone 6 than the Amaco Velvets, and I never have color shifting issues with clear glaze. They're usually cheaper, too. Just water them down as needed.
Speedball pinks, purples and reds (plus some other colours) use a cadmium inclusion stain so they don’t have the fading that underglazes with chrome tin stains do.
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The zinc and chome are tied up in the glass matrix, if the glaze is stable.
If you still have concerns there is also the option to supply the chrome with a stain. This type of stain is a spinel stain, the chrome is bound with silica and is very stable. Stains are suspended in the glaze matrix, they don't dissolve. I don't know what is available on your side of the pond, over here there is Mason #6209. Then the glaze would have the zinc + iron which isn't an issue either.
44 minutes ago, Beccap said:Re: cobalt, I have tried it with 1% cobalt and it works so will go back to that.
Might be okay with even less than that, same with the chrome. A couple progression tests would show you how much you could reduce both colourants.
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1 hour ago, Beccap said:
Would anyone know a situation in which the chrome in the glaze would convert to the more toxic one?
Nope, trivalent chrome does not convert to hexavalent chrome in glazes.Re zinc chromate, according to Hansen "Chrome in zinc glazes tends to form the stable crystalline compound, zinc chromate (ZnCr2O4). which is brown." from this link.
I plunked your glaze recipe into the glaze calc program I use but I'm not confident in the accuracy of it since specific types of potash feldspar, talc and china clay aren't mentioned in your recipe. Using generic analysis the formula is on the low side for silica though. I would do a lemon slice test on it and see if the colour changes., also keep a test piece in the dishwasher, if you use one, for a few months then compare that to an unwashed piece.What is really important is to have a stable base to prevent leaching. With colouring oxides, aka transition metals, less is better.
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Hi Jeanne and welcome to the forum.
It might be the glaze that is causing some of the colours fading out. For example if it's the pinks and purple tones fading out it could be too much calcium in the covering glaze causing it to be inhospitable with the stains used in those colours. Could you give more info or post a picture of the colours that fade out and any glaze info you have.
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I've been using slip made with magic water and my dry claybody for years. I use a smooth white claybody and fire to cone 6. I think it makes a difference which claybody is being used, some are far more prone to handles etc popping off than others.
Magic water helps as the slip made with it is stickier than made with just water, it dries harder than plain slip and during the firing the extra sodium in the slip helps flux the slip a little more than plain slip making a stronger join post firing.
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Let's not throw the baby out with the bathwater here. The form of chrome used makes all the difference in the world in regards to toxicity.
Potters use chrome oxide, it is insoluble in water and gets bound in the glaze matrix in a balance glaze. It is volatile in the kiln so it can cause a pink blush on tin white glazes. Wear gloves if you get contact dermatitis with it. (it's a good idea to wear gloves whenever handling raw materials and glaze slurries to avoid skin reactions)
We do not use the exceedingly dangerous hexavalent chromium in ceramics. Iron chromate is toxic, stay away from using it.
Zinc oxide fumes from kiln firing shouldn't be breathed in, zinc can become volatile above around 850C/1560F and like all the materials we use in glazes practice good studio hygiene and wear a respirator while handling dry materials. In a glaze it isn't a problem.
Chrome + tin pinks and reds are perfectly safe, whether in a stain or from the oxides.
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I prefer to use the term "durable" rather than food safe when talking about glazes. There isn't one agreed upon definition of what makes a glaze "food safe". In North America if a glaze does not contain lead or cadmium it can be labelled food safe. This is a very wide definition and adding something along the lines of what Mayco says "Finishedware producers bear responsibility for food safe testing their ware being sold into commerce." IMHO isn't good enough. Like Callie said Strontium Crystal Magic (SCM) is fine on the outside of pots where your mouth or food won't be in contact with it.
Re lithium and toxicity, just adding a little bit. Topic of lithium and toxicity has come up in many discussions in many forums etc over the years. My impression of the reading I've done is it might not be an issue for most people but for those on lithium therapy it can be, both in the process of mixing glazes and leaching glazes. Poison is in the dosage scenario.
With all due respect to Callie I found SCM quite rough, dry and unmelted. You could try it by itself on a test tile and see how you find it but it's a bit of a moot point because it isn't used by itself. The overload of titanium is what makes the crystals/variegation when layered with other glazes. A good way to get interesting glaze reactions is to layer 2 or more dissimilar glazes, what one glaze lacks or has a surplus of reacts with the other glaze, this often results in interesting visual texture. SCM is loaded with fluxes to react with other glazes, that's part of the reason why it works.
Re silica and glaze durability, having as much silica and alumina in a glaze is one of the ways to ensure a durable glaze. For cone 6 having at least 2.5 molar of silica is a good starting place, if the glaze can dissolve more then it's a good idea to add it if you are looking for a durable glaze. If you look at the screenshot above in the bottom half of the image you can see the silica is 1.68 so at the bare minimum it is about 1/3 short of where it should be. Overloading a matte glaze can quite often turn it into a gloss glaze and really oversupplying it can turn it back into a matte and crazed glaze so there are parameters within which to work.
Another aspect of silica that is often overlooked is the mesh size of the silica being used. When you mix glazes using a 325 mesh size silica more of the silica will likely be dissolved in the glaze than if you use 200 mesh. (not applicable for the SCM)
One more thing is to look at the colouring oxides used. Don't use crazy amounts of colouring/transition oxides, use the least you can to still achieve the effect/colour you are after.
There is a good article on toxicity that includes materials here if it helps.
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19 hours ago, StudioClover said:
Glaze fired over 12.5 hours, I have a manual kiln with a kiln sitter and have fired quite a few times now and haven’t had this problem.
What cone are you glaze firing to? How long did the firings take in the past when you didn't have this problem? Do you use witness cones or just a cone in the sitter? How big is your kiln and were the blistered pots in any specific area in the kiln or throughout the kiln? Which claybody are you using? What's different this time?
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32 minutes ago, Lbauer12 said:
but are some glazes just not compatible with certain clays even though it says they are?
Back to the jeans analogy, CK jeans fit differently than Levis. Glaze has to fit the clay, not all glazes fit all clays.
Underfiring a glaze can be a cause of crazing. If the glaze can be fired hotter then I would suggest doing it, might help with the crazing, have to test everything with ceramics. Make up some large test tiles or pieces and try out your glazes with those rather than "real" pots.
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49 minutes ago, Lbauer12 said:
I have one more question. My kiln is in my garage. Is it ok to fire when it’s around 40 degrees outside. The thing I’d be concerned about is when I take it out if the change in temperature would damage the glaze. If I wait until it’s room temperature does it matter if it’s a little cold?
No worries on firing when it's 40F outside.
If the glaze crazes when you remove pots from the kiln it won't be because of the temperature being a little cold. It will be because the glaze is too small for the clay. Simple way to think of this is to think of a pair of jeans and how they fit. If I wear a size 10 and my daughter wears a size 4 if I put her jeans on the seams will split. If she puts my jeans on they will bag out and fall off. Tight splitting jeans is like a crazing glaze, loose falling off jeans is like shivering glaze. Crazing is far far more common than shivering.
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First off have a look at the materials and their amounts in the recipe itself. I re-totalled the recipe from the original 60.60 to 100 as that is the standard base amount. When we apply a standard amount it makes it easier to notice possible areas of concern. Top part of the screenshot below is the Glazy recipe re-totalled to 100.
First thing that jumps out is there is no silica added to the recipe. Yes, there will be silica in the formula from the feldspar etc but probably not enough. (more on this later) Second thing that jumps out is the amount of titanium in the recipe, nearly 12%, this is a huge amount! Third thing, is approx 40% feldspar for a ^6 glaze is getting quite high. Put these things together and you have a recipe that won't be durable (low silica), probably won't melt properly at cone 6 and is overloaded with titanium.
I know this glaze and the "warm" version of it are very popular at cone 6. It's important to note also that it isn't used as a standalone glaze, it is used as a layering glaze with one or more other glazes. What the combined chemistry is of the layered glaze is what needs to be looked at, will those be durable, I don't know, each combination would need to be looked at.
BTW, giving credit where credit is due, this glaze started off as a Tom and Elaine Coleman ^10 glaze called Yellow Crystal Matte. Barium was removed from original glaze and strontium subbed for it, the gerstley borate was subbed with the Ferro 3124 and the zinc removed.
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I prefer throwing soft clay and more often than not my clay is too firm so my go to storage is to first pile boxes of clay in the corner of a hallway. I open the bags up and add a bit of water, do them back up and let them sit in a large rubbermaid bin filled with water for at least a few days, I add more of bags of clay into the water filled bin as I use them up.
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14 hours ago, Merve said:
Hi. I read your question cause I am struggling the same issue. I use standard low fire white slip 04 or 05 cone bisque and 06 cone duncan clear glaze. But my items come yellowisj. I am fed up with this issue please help me.
Hi and welcome to the forum.
From Standard's website and their Lowfire Slip page there is a link with glaze recommendations. Duncan IN1001 "slightly yellow, may blur crisp underglaze lines". There are four other Mayco glaze result descriptions there also. It seems the least yellowing recommendation is Spectrum 700. Standard recommends doing both bisque and glaze to cone 04.
Link here: https://cdn.shopify.com/s/files/1/0039/8177/0841/files/105-LFW_Glaze_Rec.pdf?v=1674848426
- Hulk and Bill Kielb
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I've seen a birdbath basin made a slab draped over a really large styrofoam wreath base. I haven't used either of the claybodies you mentioned but just by looking at the specs I would lean towards the Laguna WC611 given the porosity is lower. Whichever way you land up making it compress the heck out of it.
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Are you in an area that has freezing temps during the winter and would the birdbath be left outdoors year round?
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Copying a design versus using the same technique while making a pot are two different things. Are there any new designs in functional ware that haven't been done before by someone somewhere, I don't know but I sort of doubt it.
I have noticed quite a few makers doing exactly what you describe; leaving the mark of the making process visible at the joins. I don't think you need to waste your energy trying to justify your work. Getting inspiration from others work is one way to challenge yourself into making new forms, and making them your own. I'm sorry you are going through this, it must have been upsetting to you.
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Is it only commercial glazes you are interested in? If you are firing lowfire you can still use a commercial raku glaze then stain the crazing with calligraphy ink (or china paints with a subsequent very low firing).
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56 minutes ago, Marilyn T said:
I recently purchased a box of M390 and threw some test bowls.
Tagging @Callie Beller Diesel on this in case I'm misremembering but I believe she fires 390 to cone 7 for absorption reasons.
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It seems like you are going through an inordinate amount of glaze testing to solve what really appears to be a clay problem. Have you tried contacting Laguna with the batch number and asking them if they have any thoughts on this?
Is using a different red clay an option? @Callie Beller Diesel uses Plainsman M390 very successfully, is this an option for you? Could you use the clay you bought for another purpose?
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Hi Kylie and welcome to the forum.
Thread linked below from 2022, same kiln but brand new and with furniture, bought for 550. For a manual kiln 950 seems a bit steep.
https://community.ceramicartsdaily.org/topic/28072-cress-electric-kiln-fx23-p/
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750 ml yogurt container, so I guess that would be nearly as small as the paint cups with the handle. It sits in the splash tray (Bailey wheel), don't keep any sponges or chamois in it. I start with warm water then top it up as the day goes along. After a few throwing days I put it to one side, stir a splash or two of peroxide and then it goes into the reclaim. 3 yogurt containers used in rotation.
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I think that since so many of us fire to midrange the go to mindset is bisque low, fire higher to glaze. Problem with this when working with earthenware is more often than not the commercial glazes are rated in the 06-04 range yet many earthenware bodies will actually mature higher than this and still be porous enough to take glaze well. I haven't used the body in question, but if it can go to 02 (as the lit suggests) then I would at least try 03 and see how it does.
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11 minutes ago, Babs said:
My point was, given Min's info on clay, at a bisque to C03, your clay would be not very porous do would not take up much glaze if dipped,
Approx 6.5% at cone 06. I would do a test firing to cone 03 and include some test tiles. Just like the kiln firing it’s going to take some testing to see how the glazing. I’m not a big fan of underfiring clay bodies. I couldn’t find any data on absorption figures for anything other than 06, would be something I would test.
Watercolor painting on greenware clay
in Clay and Glaze Chemistry
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I’ve got a list of the Speedball underglazes that contained cadmium somewhere, I don’t recall green being on it. Since there are cadmium inclusion green stains now using Jeff’s method might be an option if anyone needs a green but uses an incompatible glaze for an underglaze that includes chrome in the stain.