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Min
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Posts posted by Min
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I've got an altered cone 1-3 glaze from Charlie Cummings who came up with it from Linda Arbuckles glazes.
Given the variability of materials and firing conditions if you try it please just try a small test amount first.
Cone 1-3 Majolica White
Gerstley Borate 10.30
Nepheline Syenite 8
EPK 5.6
OM4 Ball clay 4.5
Whiting 9.75
Silic 24
Ferro Frit 3124 37.5
Zircopax 12
The alumina in your glaze is really quite high for a lowfire glaze, it's probably why you aren't getting a good melt. (alumina stiffens a glaze)
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Hi and welcome to the forum!
Are you looking for a glaze for functional work? If so is your clay mature at cone 1?
Post the recipe you are trying to alter.
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Amaco doesn't actually say that their Bisque Fix works for cracks. From Amaco's website, "This white paste is an excellent mending material for bisque that can be used to bond pieces together or fill up broken cavities."
Different stresses within the claybody with cracks than reattaching a knob or filling in a depression/cavity.
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Good spotting the green at the bottom glaze edge Babs. Does suggest a thickness issue could be the cause.
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7 hours ago, Ron Roy said:
Does anyone have any idea if they are activley looking for potash spar?
@Mark C., wondered with your distributor status if you had heard anything?
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One more issue that could come up is how the already fired pots take to a second firing. Both the clay and the glaze getting that much more heatwork could very well have an effect on both.
- Kelly in AK, Elke and Rae Reich
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25 minutes ago, Pyewackette said:
You don't need to test the oxides. You need to test the GLAZES, each and every one
That is the exactly what lab testing does. If you follow the link I posted above for BSC you can see how they request a glaze fired sample pot, this is what they test, the fired glaze. They test for whichever oxides you pay for. Example would be if you add some stain that contains vanadium you request they test the leachate for vanadium release. It is the vanadium oxide they give you results for. Since in North America only lead and cadmium need to be below certain government approved levels on dish ware the other oxides are usually compared to drinking water levels.
Many potters, most likely the overwhelming majority of us, either use a liner without any colouring oxides to be of concern or use one with very low levels of a colouring oxide. I have had a few glazes lab tested at BSC. -
As Kelly said, options for having absolutely nothing to worry about in a well balanced stable glaze would be either zero colourants or just zircopax or iron.
If you want to use a stain or a colouring oxide and want to ensure it’s safe then lab testing isn’t going to break the bank. It’s about $35 per oxide to get tested and piece of mind.
BSC testing link https://bsclab.com/pottery-testing
Home testing, using an acid such as household vinegar or lemon/lime and dishwasher testing don’t rule a glaze “in” as being suitable for food surfaces but it will rule glazes out.
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1 hour ago, Ron Roy said:
Still don't understand how to send direct to you
To direct message someone click on their name or avatar (on the left side of their post). This will take you to their page. There is an icon of a small envelope near the top, click on that and follow the prompts.
If you have problems myself or any of the other mod’s are happy to help.
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Hi and welcome to the Forum!
How wet or dry is your clay? If it's on the dry side it could just be the water of plasticity (WOPL) is too low. Can you take a coil of clay about the thickness of your finger and wrap it around your finger without it cracking?
A photo of the cracking might help.
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2 hours ago, Pyewackette said:
Mason stains instead of things like cobalt and chromium and copper, but turns out there's no way to be sure of what they used to make the stains.
Mason puts out a Reference Chart that lists all the ingredients in each of their stains.
- Hulk, Pyewackette and PeterH
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17 hours ago, Babs said:
Will that 3rd firing cause more shrinkage?
Nope.
A third firing to ^03 can definitely change the look of glazes fired to mid or high fire. In effect what you would be doing is a strike firing which can alter glazes high in colouring oxides, particularly with iron or copper.
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Hi and welcome to the Forum!
If you don't like the results you have so far then there isn't really anything to loose by trying the wash. As to whether it covers the glaze, probably not but something might happen, what happens is going to be trial and error. Washes can be applied then wiped or sponged off green or bisque or they can be brushed on either on top or under a glaze. I would suggest warming the piece(s) up before trying to apply the wash. If you use a copper oxide wash overtop of a glaze it would be best not to do this on a surface that could come in contact with food.
Re your muddy underglaze colour with clear glaze, this can happen depending on the chemical composition of both the underglaze and the glaze insofar as the reactions between ingredients. For example green underglazes that use chrome (in the stains within the underglaze) will turn the green to a not so attractive brownish colour if the glaze contains zinc, chrome tin stains (for some pinks, reds and purples) will get bleached out if the glaze doesn't have really high calcium levels causing grayish tones to the underglaze. Colours can also be affected if the glaze is applied too thickly, underglaze can look milky and clouded.
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On 11/19/2023 at 7:54 PM, VermontPotter said:
I'm unsure if the inside of pinholes are porous at all.
If you do an absorption test on your clay it will tell you this. It's a good idea to test a claybody for absorption using your kiln and firing methods rather than what a clay manufacturer publishes it as. For functional ware below 1 - 1.5% is usually good. Link here on testing absorption if you need it. If the clay isn't porous for all intents and purposes and you run your pots through the dishwasher I wouldn't worry about the odd pinhole. Aesthetics are a different point, a pinhole on a pristine porcelain piece is going to stand out far more than one on a rustic piece.
Ryan Coppage article here discussing bacteria growth on crazed glazes. Conclusion is if your clay is vitrified and the pots run through a dishwasher bacteria is not an issue. (article is behind a paywall but you can access 3 free articles a month) I haven't come across any literature proving pinholes harbour more bacteria than craze lines.
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Glaze crawling is usually from one or two main causes. Either the glaze was too thick or the bisque was contaminated with dust or oil (from skin or hand lotion etc). Crawling can also happen when the recipe contains too high an amount of a material with excessive shrinkage but it doesn't sound like thats it in this case.
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We do our own kiln repair but there is something that came to our attention regarding installing kilns. When we were shopping around for a new home insurance policy quite a number of years ago most home insurance providers wouldn't insure me unless I could prove a licensed electrician did the direct wiring for the kilns.
I don't think the "Potter" designation means you have to do absolutely everything. Are we ruling out people who use pre-made dry glazes, bottled glazes, bought clay, etc. Slippery slope.
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Yup. A gold and luster eraser.
https://tuckers-pottery-supplies-inc.shoplightspeed.com/gold-luster-eraser.html
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15 hours ago, Lauren F said:
I fire these to cone 6, in a pretty fast firing. The kiln is on the larger side (https://coneartkilnsshop.com/index.php/product/2827d/)
Yes, I could slow cool it.
Thanks for answering a couple points, makes it easier to give suggestions when we have more info.
I would glaze fire the tiles on waster strips of clay. These don't need to be bisque fired if they are bone dry. For waster strips I would roll out a slab of clay (same clay as the body) to about 1/8" thick and about 19" X 16" then cut it into strips about 2" wide. There can be gaps between strips but have the edge ones fairly close the edge of the tiles. Make the strips a bit thicker if you plan on glazing the edges of the tiles right the way down with no wax line. Extruded coils would work too. Reason for the waster strips (or coils) is twofold, one to allow the tiles to shrink / move freely without getting hung up on the shelves and secondly to allow airflow under the tiles. During cooling the shelf is going to retain heat much more so than the air will, this means the bottom of the large tile can be much hotter than the top surface which can lead to cracking.
I would place the tiles in the middle section of the kiln. If you use half shelves make sure they are lined up perfectly. I would also suggest ringing the tiles with either other pots or square or triangular kiln posts laying on the side, this also helps even out the heating and cooling.
I know you said the tiles are time sensitive so you might not have time to dry waster strips / coils, if so some people have success with using silica sand under large slabs, sculptures also.
To be super safe you could also add a slow cool through the quartz inversion zone. (slow from 1100F - 1000F, tc's measure air temp not shelf temp so the 100 degree range is pretty safe) Don't open the kiln until it has cooled down well below 400F. I would also glaze fire them slowly, not a fast firing.
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@Lauren F, what cone are you firing the tiles to, cone 6? How are you firing the tiles? How big is the kiln, room to put a shelf above and below the one(s) that will have the tiles? Can you slow cool the kiln?
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If the colour is the same in both (I'm guessing it is if you haven't noticed a difference) then put a drop of water on a piece that was for sure bisqued to 1860F and time how long it takes to soak into the clay. Repeat with the clay that was possibly only bisqued to 1200F and see if takes longer. You can also stick your tongue on both pieces, the lower fired one will stick more. (maybe don't do this in public)
Strange blow outs during bisque fire
in Clay and Glaze Chemistry
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Hi and welcome to the forum!
Could you post a picture of the blowouts? Also, where in the kiln were the large bowls were in relation to spyholes?