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Min
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Posts posted by Min
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I wonder what would happen to it if you scraped some into hydrochloric acid? Interesting.
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There isn't a one size fits all firing schedule to fix pinholes and blisters but somewhere to start would be to drop 100F below your top temperature and hold there for 15 minutes. One other thing I do is for my kiln and my glazes it works to fire below the top temp and soak there to bring cone 6 down then do the drop and hold/soak, (I also do a slow cool for my satin matte glazes but that isn't necessary for many glazes). It can take a few firings to dial in what works for your kiln and glazes.
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On 3/2/2024 at 1:01 PM, chris123 said:
I'm attempting to transition from fully thrown plates to partially slab constructed to save time and effort. The first iteration of the plates are simple with a flat bottom, no foot, and a short turned up (nearly vertical) side about 3/4-1".
I'm seeing a lot of plates made like this these days, it's a good way to make a plate with this type of design. I've got one bisque fired as a test but haven't got it glazed yet, it was really fast to make and I also like that it doesn't require any trimming. That nearly vertical edge is going to help prevent sagging/warping during the glaze fire compared to a flatter more horizontal rim.
I would suggest putting one of yours in a glaze firing with no glaze on it, if it doesn't warp then it would suggest the glaze fit could be the problem.
Welcome to the Forum.
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I stumbled across this Etsy listing where someone is selling feldspar granules, does this help? I know some people use chicken grit (not the oyster shell type) for big chunky inclusions in claybodies also.
@PeterH, do you know of anywhere in the UK to buy it?
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Have you tried contacting them and asking if they have any other advice?
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From the article linked below from Charlie and Linda Riggs, they put the pot within minutes of applying the slip into the kiln and not to allow the slip to dry at all. Is this the way you are doing it also? I wonder if adding gum to the slip would help?
(behind a paywall but can access 3 free articles a month)
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Bloating for sure is caused by overfiring but then there is also blebbing.
I think that in the last 20 years or so the term bloating is used to cover both bloats and blebs but with pyroplastic claybodies, such as many of the smooth white bodies, an air bubble within the clay wall will expand during glaze firing when the glaze has melted enough to seal the surface of the clay and the air pocket expands causing a bump on the surface of the clay. These don't show up during the bisque but do show up during the glaze fire, does't have to be overfired to happen.
- Babs and Kelly in AK
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Bendy foam hair roller from the dollar store, I use them for supporting the rims of slab pots etc and also around the handles of the glaze tongs. I slit them up the side then tape them on, saves rubbing the skin sore between my thumb and index finger on a long glaze day.
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5 hours ago, Vik said:
I'm wondering if it's possible to streamline this process by adding the 10lbs of water directly into the original bucket of low glaze and starting the mixing and sifting process from there.
Yes.
Most glazes are just fine with running through an 80 mesh screen once, some run it through twice. I hold a bit of the water back to rinse the sieve out at the end of sieving to try and get as much out of it as possible.
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On 2/25/2024 at 9:18 AM, mrcasey said:
I appreciate your reply and agree with you, but do the manufacturers state anywhere that this is ok?
It seemed like a given that they can and many of us do this but I reached out to the Ceramic Shop to ask them specifically. They make, sell and wholesale posts to other ceramic suppliers in the US and elsewhere. (they make triangular and square ones)
Their reply:
"Thanks for reaching out!
To my knowledge, yes, they can be used on their sides. Kiln posts are made to be the most stable on their top ad bottom though and they will take up extra room in the kiln on their side. If you need a smaller kiln post for some works you can check these out: https://www.theceramicshop.com/product/148/kiln-post-1/
All of our kiln posts are rated to cone 10.
Best,
Syd" -
Hi and welcome to the forum.
Somewhere to start might be looking at Terra Sig with added materials such as salt, soda ash, oxides and stain and borax. Washes would be another avenue to experiment with. . Also have a look at Mary Fox’s work where she uses lithium carb plus copper for a dry surface. (on some of her lowfire sculptural work)
This articles from Pottery Making Illustrated might be a good place to start for alternative surfaces. It’s behind a paywall but you can access 3 free articles a month.
Perry’s wash recipes are for cone 10, for lower firing adding flux would probably be necessary for some of them. Also, firing in oxidation rather than reduction will make a difference with some of them.
http://shambhalapottery.blogspot.com/2013/03/happy-st-paddys-day.html
Pit firing would be another avenue to explore.
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From the Kanthal website, they recommend an oxidation firing to 1050C / 1922F for 7 to 10 hours. I'm thinking the people who hold for 7 to 10 hours are going to be few and far between if they mean a hold at 1922F and not a slow rise up to this temp. I typically hold for 3 when replacing elements. I don't know anybody who does a re-oxidation of the heating elements. I guess they are talking about a repeat empty kiln firing?
edit: Euclids recommends holding for "several" hours also.
"To protect elements used under these conditions there is an element conditioning process that is desirable for FeCrAl elements, which requires heating the elements, in air, above 1830oF/1000oC, and holding for several hours. This develops an aluminum oxide coating on the outer element surface, which can dramatically improve element life.https://www.kanthal.com/en/knowledge-hub/heating-material-knowledge/operating-life-and-maximum-permissible-temperature/
"In such cases Kanthal® alloys are recommended, provided the heating elements are pre-oxidized in air at 1050°C for 7–10 hours. Reoxidation of the heating elements should be carried out at regular intervals."
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There is an old thread linked below where a few different methods are used, including tattoo ink and china paints.
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Another thing that works for rims instead of a piece of chamois is the microfibre cleaning cloth that comes with eye glasses. They last a long time and don't go slimy at all.
Wire coat hanger corners to make callipers.
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@s6x, could you post who the maker is?
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I've altered the Studio White to remove the Gerstley Borate and supply the boron with frits instead. I've also removed the Minspar 200, I did this because I wanted to get some clay into the recipe and the original only had 2 bentonite and zero epk, most of the sodium and potassium is now supplied by a frit. I replaced the calcium carbonate / whiting with wollastonite for two reasons, firstly it melts better than whiting + silica and secondly it has less loss on ignition. I kept the oxides the same in the formula, they are just being supplied from different materials. My hunch is it will melt more than the original. Kept the same flux ratio and reduced the LOI. Because of all the frit this glaze should have 2 main melting zones, lower temp for the frits to get started melting and then higher temp for the zinc. (put the epk in your mixing container first so the frits don't just stick to the bottom)
For the Wollastonite Clear I used 3 frits, I kept the boron level high. I wanted to reduce the silica and alumina amounts plus raise the silica above a 1:10 alumina : silica ratio. I reduced the sodium and potassium and increased the magnesium. By reducing the alumina and silica this version should be more fluid than the original, it did raise the COE slightly though.
Overall the Wollastonite Clear should make a more durable glaze than the Studio White. I didn't add tin to any of the recipes, wanted to see the numbers without it. (use 325 mesh silica) Please ask if you have any questions about why I subbed what I did if you are unsure of anything. (as always please test a small amount of the altered versions, what looks okay on paper doesn't always translate to a pot)
edit: "I'll do a full cone 6 fire on a small bowl as soon as I bisque some test bowls." +1
- Marilyn T and Kelly in AK
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I would keep the kaolins, can sub those quite easily in glaze recipes. Calcined kaolin I would keep, use it as part of the kaolin in any glazes that crawl due to high kaolin amounts. (have to recalc the amount used to sub for un-calcined kaolin)
Alumina oxide can be used in place of alumina hydrate in kiln wash (use 1/3 less by weight).
If anyone uses washes then the Barnard, Burnt Sienna and Umber can be used in those if they want dark/earthy colours.
Kona (F4) can be subbed for Minspar 200 in a 1:1 ratio.
Tennessee Ball comes in different forms, given how inexpensive ball clay is I would donate that one if you don't know which you have. Fireclay, Sagger clay and mullite, unless someone is making their own bodies I'ld donate those. Redart can be used in bodies and glazes.
Silica sand, might be useful if someone is firing large heavy pieces or wants to wedge into their clay but if nobodies used it for years maybe not worth keeping.
CMC gum, mostly used for making brushing glazes or hardening a dusty glaze.
Calcium Carb precipitated, this is just whiting/calcium carb, keep it.
Black Iron Oxide, can be used in glazes, need to compare the iron content with whichever red iron oxide you are using to sub it. It doesn't create the red staining mess that red iron oxide does.
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Firing to a full cone 6 is worth doing. This might just be enough to rid the glaze of the dimples.
I'll rework the recipes so you have an early melting one and a late melting one, this way if the glaze is sealing off the clay before it's finished offgassing there might be a difference in how well they heal over the pinholes. I'll use the Studio White as the base for a late melting one, zinc melts later in the fire and as it's an auxillary flux in the Studio White it might work to do a modified version of that one. Downside is zinc itself can create problems with pinholes so it might be counterproductive. What is it about the Studio White that you don't care for it as much as Wollastonite Clear?
If you have any flat test pieces you could use as tests I would use them, pinholes are harder to fix on flat surfaces.
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If the dimples are from unhealed pinholes then it's logical to think that giving the glaze more heatwork will smooth them out. It's interesting that the refired bowl smoothed out the pinholes and the clay didn't bloat. What happened when you fired the Wollastonite Clear to a full cone 6, was the clay bloating or had a higher absorption? At ^7 you had increased absorption, correct?
Common fix to make the glaze more fluid would be to remove some alumina and silica but keep the alumina:silica ratio above 1:10 but that's going to mess with your COE and you might then get crazing which in turn would need some of the higher expansion sodium and potassium being replaced with a lower expansion flux such as additional magnesium or lithia, at this point so much tinkering will have been done that it will basically be a different glaze. I can alter your recipe to do this if you want to give it a go. Do you have Ferro 3110?
Do you have the recipe for the Studio White? Have you noticed if different thicknesses of the Wollastonite Clear results in more or less pinholes/blisters? I also think the pinhole issue is from the claybody but understand you are invested in using this body for now so it looks like glaze formula + firing + glaze fluidity + glaze thickness are all going to play into trying to overcome it.
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2 hours ago, Mudfish said:
So, my next question, are some slips more compatible with various glazes than others ? Also, is it better to formulate slip/colored slips using my clay body slurry? This seems like a solution to me
Yes to both questions.
I would suggest just putting that 5 gallon bucket of glaze to one side for now without trying to adjust it. Make up some slip with your claybody and try that with your colouring oxides or stains. Your altered glaze might be just fine the way it is.
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Have you tried the Rutile Green base with 5 tin (and no rutile or copper)?
Drop and Soak program for Cone 6 in Skutt
in Clay and Glaze Chemistry
Posted
@Potpotpotter, slow cooling is different from a drop and hold/soak. With slow cooling the preprogrammed slow cooling schedule will be something like 9999 (freefall) from the last temperature in the program down to 1900F then 150F down to 1500F. If a glaze is already a matte without this slow cooling then by adding one you could very well end up with it being far more matte. This schedule is a basic slow cool for functional glazes, many people do their own custom slow cool schedules to suit their glazes. Slow cooling promotes certain glaze characteristics such as micro (matte glazes) or macro glaze crystals or visual textures within the glaze. It helps with some glazes and does nothing for others. Slow cooling is also useful for large pieces to help prevent dunting/cracking.
A drop and hold/soak is helpful to help heal pinholes and blisters. The temperature you hold at will depend on how fluid the glaze is at certain temperatures. There are many causes of glaze pinholes and blisters, glaze thickness, application, glaze chemistry, bisque and glaze firing all can contribute to them.