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Min
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Posts posted by Min
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Hi Peter and welcome to the forum!
Yellow Banks 401
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The peeling underglaze is caused by the application, either too thick an application or underglaze itself was applied when it is too thick. For solid coverage it's best to apply several thin (water down the underglaze, most need this right out the jar) coats rather than one or two thick coats. Each layer needs to be dry before the next layer is applied regardless of whether the underglaze is applied to greenware or bisque. I prefer to put the underglaze on nearly dry greenware then bisque then glaze.
Is the glaze crazing in addition to the underglaze issue?
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9 hours ago, PeterH said:
... although in this case you could reduce the cost by casting a thin coloured layer backed by a thicker white one. (But might have issues where the white shows.)
+1
this is what I did for tumblers
- Callie Beller Diesel, Babs and PeterH
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4 hours ago, GEP said:
What would you recommend? My local supplier is also having some issues with Frit 3134. It’s marginally cheaper, but still not cheap. Right now, they keep a waiting list of everyone who wants it, and anytime they receive any, it’s sold out before it hits the shelves. They told me I can get on this waiting list too, if I want. Are there other options?
This really depends on what else is in the recipe.
I took a sample glaze, Lynette's Opal, this seems to be widely used for the past couple years and replaced the Gerstley Borate with 3 different materials that all could be used to supply the boron, Ferro Frit 3124, Laguna Borate and Ferro Frit 3134. Each of those contributes slightly different amounts of other materials than Gerstley Borate. What looks okay on glaze calc doesn't always translate to the bucket or glaze fire so testing would be required to see how they behave. I use Insight glaze calc but the same thing can be done for free using Glazy.
I purposely used Ferro 3134 in one of the examples and Ferro 3124 in another, see what it did to the EPK amounts? Since Ferro 3124 supplies more alumina than Ferro 3134 I had to reduce the EPK in the former. Since the original had Gerstley and we know that contributes to glaze gelling the alternative recipes with frit will suspend less therefore some extra bentonite might be necessary with Ferro 3124 and Laguna Borate version.
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1 hour ago, neilestrick said:
Could be from the fireclay, though, or some sort of contamination.
@Clay17, I recently ran into a clay that was using Lincoln 60 fireclay in the body and it has a lot of stuff in it that didn't pass through a 60 mesh sieve. I made some slip from a new chunk of claybody (right out the bag then dried and slaked) through a sieve then rinsed it under tap water to see how much there was. Don't know if there is Lincoln 60 in the 630 clay or not but it's a quick way to check for grit.
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18 hours ago, Mark C. said:
Better stock up now as its going to go away is my thought
I wouldn't at the prices being charged. There are lots of other materials that can supply the necessary boron for far less money.
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1 hour ago, Neil Fallon said:
She described the material left in the bucket as “beach sand”.
What mesh size is the sieve and the silica?
1 hour ago, Neil Fallon said:Yes, if the piece is dipped and left in the glaze too long, the glaze cracks as it dries on the piece. It also crawls on pieces where the glaze was applied too thick.
Going forward if you have a glaze recipe with a large amount of kaolin like this recipe does then calcine part of it. Just a ballpark amount would be to have up to 20% kaolin and anything over that use calcined kaolin. (just put the kaolin in a shallow bowl and add it to your bisque fire) Calcining removes the chemically bound water from the kaolin so it will weigh less and reduce the shrinkage of the glaze thereby reducing cracking and crawling than uncalcined kaolin. Since the calcined kaolin has less water in it's weight you need to allow for this in the recipe. Explanation on how to rebalance the recipe here if you need it.
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5 hours ago, GEP said:
It’s weird that Laguna is being transparent about Spodumene and Lithium, but not Gerstley Borate.
It was the Spodumene Substitute that I emailed them twice about and didn't get a reply. My email was forwarded to a tech and that was the last I heard from them. I then contacted Pottery Supply House in Ontario (same company as Euclids) and asked about their Spodumene Blend and they confirmed it was the Laguna Substitute product and quickly gave me the analysis which I posted in the thread above that Mark linked. Still haven't heard anything from Laguna.
We've been down this road before with Gerstley, lots of subs already developed, I think it's just a question of finding one and tweaking the chem to use one of the alternatives. Quick google search, Laguna Borate at Sheffields, $34- for 50 lb bag. Main differences are Laguna Borate has more boron, alumina and silica.
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4 hours ago, GEP said:
This year it is $250/50 lbs.
It's $516 / 22.68kg here, so a little over twice what you paid.
I've contacted Laguna twice in the past month asking about an analysis for a different material, still haven't heard back from them. Call me sceptical but I doubt they would disclose any info on the Gerstley Borate situation.
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From the Nov 2018 CM magazine Carrie Wiederhold has an article on making a grid firing fee box that looks pretty efficient. It's behind a paywall but you can access 3 articles a month for free. Image below taken from the article to give an idea of it. Article explains how the grid is made, works and the pricing equation.
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A few thoughts after looking at the recipe
- having only frit, epk and silica there shouldn't have been anything that didn't go through a sieve, even a 100 mesh one.
- are you confident the kiln is reaching cone 6 (Glazy has this as a cone 6 glaze)
- boron is super high for a cone 6 glaze which means the glaze probably won't be very durable
- how does the glaze behave in the bucket when you stir it? does it settle really fast and form a hard layer at the bottom of the bucket or is it well suspended and creamy?
- does the raw glaze crawl or crack if you apply it thickly to bisque (test piece will do for this test) 35% epk is a heck of a lot, I would expect the raw glaze to crack or crawl
-are you confident in your weighing of materials and the materials you used are actually what they should be?
BTW you are logged in if you are able to post, did you have another account or name you logged in here previously with?
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One from Glazy below. I have found that with dark glazes it is hard to see crazing, there is a thread here where @Pieter Mostert suggests "Fill the vase with a mixture of vinegar and Whiting (it will bubble) and let it stand overnight. The Whiting and vinegar react to form calcium acetate, which is soluble, so will migrate into the body if it allows water to seep through. After you empty the vase, the calcium acetate will slowly migrate to the surface as the body dries out, and will concentrate along any craze lines." If you try this and it works it would be great if you share your findings here as there isn't much info on this.
I've also read about Ron Roy accidentally highlighting crazing with steam. Apparently he had a bowl of hot mashed potatoes and the steam from the potatoes showed in the craze lines. Don't know if it was a matte glaze or not.
Another thing that helps with testing for crazing on glazes which aren't dark is to use Sumi or calligraphy ink instead of a felt pen. It seems to show crazing better than a felt pen. In the image below the author states they used ink.
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Hi Earl and welcome to the forum!
You are more than welcome to ask your newbie questions in any of the forum sections! If one of the mods thinks your question should be moved to a more appropriate section to get responses then we will move it with a link to where it's moved. There isn't one section specifically for new potters but it's very helpful to say you are a new potter when asking questions so you get answers in keeping with your experience level.
PS - I am going to slightly edit this thread title.
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Another option would be to put the full 5kg batch into a pail with a lid that can be sealed on (twist type pail lid) then roll it and flip it around for 15 minutes or so. Leave the pail sit for a day for all the dust inside to settle then scoop it out just like any other dry material. If it sits for a while then roll it around again before using. I do smaller 500 gram test batches of base glazes like this all the time using a large freezer ziplock bag.
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4 hours ago, Mudfish said:
I will try this idea also. A couple of years ago, I did try a version of your idea, here. I don't remember flipping the slab over to rib both sides, though. Anyway, the form ended up cracking all around the join as it was drying. I abandoned the idea but maybe I should have kept trying.
One thing I forgot to add was it's a good idea to place some weight in the middle of the base as the pot is drying. (I use a rectangular kiln brick and leave it there until the pot is dry) I use a very thick slip made with magic water and my claybody for the joining slip.
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I'm not sure but to me it looks like the glaze has rounded over slightly on the crack in the second, third, and part of the 4th image, perhaps the crack in that pot was there either before glazing or on the temp rise up.
A totally off the wall suggestion here but maybe something to consider. I've made dozens of oval shaped baking dishes (used ^5 clay with sand) fired in ox. which I realize you aren't doing but perhaps the forming method might be an alternative way of making them that could work for you. Instead of throwing one large baking dish for the type in the image below I roll out the base, compress both sides, and let it stiffen up to cheddar cheese leather hard. While the base is drying I throw a bottomless ring the height of the walls with a thin tapered flange on the inside. I cut the wall from the batt and when soft leather hard make the oval shape. When the wall ring is as firm as the slab base I attach the two, slipping and scoring the edges that join. Outer bottom wall edge is trimmed where it meets the base and I use a pony roller to really compress the seam. On the inside the tapered flange is ribbed onto the base. Rolled rim on the top edge of the wall to make a built in handle. I haven't made these as round forms like yours but I don't see why it wouldn't work as would adding a footring. I agree that glazing the bottom might help although I don't glaze the bottom of these forms.
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- Mudfish and Kelly in AK
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Laguna's Spodumene Substitute Analysis below. Compared to Insight Glaze Calc Materials Data the Li2O is down nearly a full percent. (Slight decrease in Al2O3 and SiO2 although those are probably not significant in most glaze recipes)
I asked Pottery Supply House in Ontario if their Spodumene Blend is the same product and they confirmed it is.
I would suggest the Li2O difference could very well make a difference to glaze fit and/or colour response in some glazes. So not only is the price of spodumene through the roof we now probably need to use a bit more of it compared to the old chemistry.
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I think I would use some sort of physical barrier like a very thin piece of wrinkle free plastic wrap.
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I just emailed them and asked for a recent analysis. Hopefully it's an isolated lot that is causing the absorption issues, or something else entirely. Just going on what I was told by a west coast clay supplier yesterday.
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7 hours ago, Kelly in AK said:
What cone is that?
6.
Yup, this is not good. I believe it's used a lot in many stoneware bodies on the Western side of the US.
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I'm in the process of testing new claybodies and one of the candidates contains Lincoln 60. Absorption figure for the clay in question is supposed to be around 0.5. I have run 8 absorption tests with this clay and have figures ranging from 2.3 - 2.66%. Just got off the phone with the company that makes this clay and the person I spoke with thinks the issue is stemming from the Lincoln 60. She said they are seeing absorption issues with other bodies that use Lincoln 60 which is now coming from the far end of the mine. Manufacture date of Feb 2023 for the clay in question.
Anybody else finding absorption issues with bodies employing Lincoln 60?
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@Bill Kielb, I'm curious about the idea of nepheline syenite absorbing lead from a lead contaminated kiln too. I did a quick Google search and came across an article from Clay Times 1995. Link to it here, third subject down from the top. Basically it says make a container to the inside measurements of the kiln, fill it with nepsy then fire at 270F / hr to 2000F then hold there for 2 hours then dispose of the nepsy appropriately. Not guaranteed to be 100% effective and a caveat to test wares with a lead kit. Would be good to see some specific data on this.
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Hi Kelly and welcome to the forum! Congrats on your new kiln.
My first thought is there might have been a programming error. What temperature did the controller say the kiln reached with the cone 5 firing?
BTW- It's actually good that you did a slow firing with an empty kiln for the first firing as it conditions the elements. Building up an oxidized layer on the elements by doing an empty kiln firing will help with element life.
- Roberta12, Hulk and KellySnider
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+1 for what Rockhopper said.
Couple other points, I've seen 2 brands of commercial underglaze transfers rub off at bisque temps, these in particular needed to go to cone 6 either with or without a covering glaze to be stable. Also, unless you fire the underglaze on the pot the water from applying the underglaze transfer could well disrupt the underglaze layer.
11 hours ago, Ben xyz said:Will unsealed Underglazes still be archival color-wise?
Yes as long as they are fired to at least the cone the manufacturer has them rated for.
11 hours ago, Ben xyz said:(a gloss would be necessary in the interior of the vessel if that's the case)
For cleaning purposes then yes I would liner glaze the pot, however a clay intended to hold liquids shouldn't weep / leak even without any glaze inside. I can't speak to non fired products used to seal the clay. If you only use a ceramic glaze on the inside it needs to fit the clay well or you can get dunting from the tension between the inside and outside surfaces of the clay surfaces.
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low fired stoneware vs mid fired
in Studio Operations and Making Work
Posted · Edited by Min
grammar
Pete Pinnell did a test of claybody types many years back and one of the things they tested for was modulus of rupture, don't know how relevant that would be to your question although it's an interesting read. Link here to it. I would hazard a guess that how you pack and ship your sculptures might be more germain.
Sorry your question didn't elicit any responses in the chem section.