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Min
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Posts posted by Min
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Ben, have you seen the work of Jeffery Nichols? He uses unglazed Velvet underglazes on the outside of his pots. Might be worth trying to contact him and asking if he has any research or further info on the durability of unglazed Velvets. Logically they are similar to a highly pigmented stained slip, just with less clay and more flux.
Pots by Jeffery Nichols
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There is a thread link below about Liquid Quartz with another link within that thread with more discussion on it.
https://community.ceramicartsdaily.org/topic/30248-liquid-quartz/
What was the definition for archival from Amaco? Seems odd that they market their velvets for use with or without a covering glaze but could not say the velvet finish was archival.
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Soy wax works great on the bottom of pots, also smells less than paraffin wax when burning off.
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@ClayFishStudio, ask your local supplier if they can bring in some empty Amaco 1 pint glaze jars. I drilled the lid to fit the critter top, depth of the jar is a little deeper but other than that they work well. When I bought mine they were about $1- each.
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Hi Paula and welcome to the forum.
Glaze blisters are one of the harder things to figure out the cause of and remedy. Blisters can be from the glaze application, materials, the firing, the claybody, the application or a combination of these. Given when you refired to a cone lower it healed most of the blisters is a good sign. Do you use cones to verify your firing and confirmed you aren't overfiring? Have you tried an unfired piece at a cone lower or only a previously fired piece? This is where I would start trying to solve this. Since we don't have the recipe(s) for the problem glazes we can't offer suggestions for altering those but the claybody, firing and application method could be areas to look at.
In this months CM there is a really good article on glaze blisters by Jeff Zamek, it is behind a paywall but you can access 3 free articles a month. It's well worth a read. Link to it here.
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3 hours ago, ChromaticCeramics said:
Hi Min, thank you... I'd like to lay down quick energetic brush strokes that are fairly opaque... I've tried speedball and velvet underglazes which seem to need 3 coats to be opaque so you can't really get that quick spontaneous brush stroke look I believe.
Oh gosh, sorry, don't know how I missed that. Okay, so plan B would be to make up a slip and use some oxides to make a black. Slip needs to go on greenware, as soft leatherhard as possible to avoid fit issues. Also test the covering glaze over it before committing a "real" pot. There are recipes for black slips that contain various amount of colouring oxides, most include iron + manganese + cobalt and sometimes chrome also. Something along the lines of this recipe from Vince Pitelka.
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@davidh4976, good job on subbing the Custer out. I like how much silica and alumina are in the formula, will make for a good durable glaze. I could see a bit of iron in the recipe if you ever wanted to tone down the blue a little.
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5 hours ago, Zuri said:
I only do 30 minutes of soaking at the moment
Are you soaking at the end of the firing? If so a 30 minute soak is going to add a lot of heatwork which could be making the clay brittle. Could you post a couple pictures, including the rims? When you say you could try a long soaking at the beginning what do you mean?
edit: for sure it's not glaze shivering?
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6 hours ago, C.Banks said:
In my limted understanding, firing one cone hotter does not involve any significant change in the size of silica.
I had a look through Hamer's Pottery Dictionary of Materials and Techniques and under the Crazing topic. Snippet from there reads "Where a glaze is overfired and then crazes it is because the overfiring affected the body more than the glaze." also "The crystalline silica in the body becomes fused silica and in this form it has a much lower rate of contraction. The glaze still has the same rate of contraction and therefore in proportion it contracts more when overfired than when correctly fired."
Hamer doesn't quantify how much overfired. I haven't seen it with a single cone higher firing, maybe more prevalent with very tight firing range claybodies?
- davidh4976 and Roberta12
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I would verify with cones. You could fire the kiln with shelves (and some extra posts for mass) but without pots to calibrate firing with the cones.
Is the design of the pieces part of the problem of chipping? Where are the pieces chipping, rims?
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4 hours ago, Bill Kielb said:
Yes of course simplicity is best.
Thank you.
I believe I made my point. If you would like to debate this further then please feel free to PM me, any of the other mods or admin.
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18 minutes ago, Bill Kielb said:
maybe you have something and firing higher is another tool to help with crazing.
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.
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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.
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On 1/31/2024 at 9:17 AM, Bill Kielb said:
just firing hotter does not necessarily mean the glaze will have greater flexibility and could actually become harder and more prone to tensile failure.
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?
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1 hour ago, davidh4976 said:
Does Frit 3124 contribute to settling/hard-panning?
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)
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2 hours ago, davidh4976 said:
Is it correct to say that if a glaze/clay combination does not craze at cone 5, then it should not craze at a higher cone firing?
Yes.
2 hours ago, davidh4976 said:I use the alternating boiling/ice water baths for craze testing.
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.
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1 hour ago, akilpots said:
i was trying out a bunch of different laguna clays as they are the closest supplier to me i found one that i liked at cone 6.
laguna RSMC
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.
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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)
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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.
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1 hour ago, Kelly in AK said:
When I learn about something I get more interested in it. It’s a blessing, and a curse. I’m always up to something.
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.
- Pres, Kelly in AK, Hyn Patty and 1 other
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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."
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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.
- Roberta12, Hulk and Piedmont Pottery
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Bertoncello's "Screziato Havana" Glaze?
in Clay and Glaze Chemistry
Posted
Looks like a cream breaking brown glaze. Sometimes called cream breaking rust or red or more recently there is a glaze called Folk Art Guild White that is the same type of glaze. High level of tin with a small amount of iron, application is crucial to get the breaking colour.