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Min
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Posts posted by Min
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Anything is possible, it might be from firing bisque with glaze but my hunch it's less probable than being a slightly yellowish glaze colour from trace iron.
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I'll let admin know, might not be until after the weekend that they get my message though.
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15 hours ago, Babs said:
Could it be the white slip turns Cream at glaze temp?
Bisque is done to cone 05, glaze is done to cone 06 or 05. Could just look at an unglazed part of the pot to see the difference in the casting slip colour.
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Hi Jennifer, have you had a look at the FAQ's for this? Link here in case you need it. https://ceramicartsnetwork.org/faq
If you can't find a fix in the FAQ's try the customer service (if you haven't already). https://ceramicartsnetwork.org/contact
Let us know if you get it sorted and welcome to the Forum.
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Sounds like you are using commercial glazes? If so there really isn’t anything except trial and error to find one that is satisfactory to you. Yellow tinge is probably from trace amounts of iron in the glaze. Might be less expensive to buy a white glaze rather than trial and error testing of different clear glazes if white is what you are looking for.
BTW, you can turn a clear glaze white by adding an opacifier, zircopax would be the least expensive one. Would need a glaze sieve, accurate scale and a small amount of testing to do this. -
7 hours ago, Hulk said:
There's a nice write up on page fourteen of this month's (March 2023) Ceramics Monthly magazine.
Thank you Tom!
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Term used for a non fluid glaze is a stiff glaze, this sounds like what you are looking for. Thickness of the glaze and over firing can also make an otherwise non fluid glaze runny or fluid.
From the Mastering cone 6 book the Glossy Base 2 Licorice recipe without the iron and cobalt and with approximately 13% zircopax is a good starting place for testing. Whether or not it fits your clay will have to be determined by firing and testing as not all glazes fit all clay bodies.- Kelly in AK and Rae Reich
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Hi Sandi and welcome to the forum.
Doesn't look like anyone yet has done this. If you do try it let us know how it turns out.
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Test tile weight results on 2 test tiles about the same thickness as my pots, maybe a tiny bit thicker.
1 - bisque weight 48.51 grams, weight just after dipping 53.39, fired weight 50.86
2 - bisque weight 54.97 grams, weight just after dipping 61.80, fired weight 58.32
sg of glaze 1.50, this glaze needs a heavy application to develop microcrystalline finish.
Glaze LOI is 6.50Yup, salty water and CO2.
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11 hours ago, Kelly in AK said:
I can’t think of any metal commonly available (as a threaded rod) that wouldn’t be ruined at cone 6, also the clay will shrink around it and most certainly crack.
+1. @Jill Smolkin, have a read through of what happens to stainless in regards to high temperatures, here is one link. There is a reason nobody does it.
Re your design, I would suggest making the base as one piece and the top as another. After glaze firing I would secure the base onto a flat surface and seal the bottom edge with a very thick coil of clay then fill the base with cement. Once the cement has nearly set insert a piece of threaded rod into it. This will give you a good solid and sturdy base with a threaded rod without compromising the strength of the threaded rod or the clay. Firing it in two pieces also allows one or other of the two pieces to be remade if there is damage to it during the firing.
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Hi Sandy and welcome to the forum.
2 hours ago, Sandy091 said:...and different boxes of casting slip.
Might be a change in the formula due to the recent talc issues. Which Millar porcelain? Do you have the SDS for it? Are you getting warping and/or slumping also? Using cones to verify firings haven't gotten hotter?
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Are the figures of 10-20% for thinly cast wares, thrown pots, sculpture or just a general average?
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3 hours ago, High Bridge Pottery said:
I made this comment in my first post but never really followed it up and I was certainly way off in my estimations of how much glaze impacts the cost. I did a little searching today and the only figures I could find were 10-20% of the bisque weight is the weight of glaze applied. I will have to test my own work but I thought it would be a lot less.
10 - 20% of the weight of a pot is glaze seems really high to me too. I have a glaze firing starting later today, I will weigh a test tile or two and then glaze and reweigh it after firing. Rest of the pots are already glazed or else I would do some of those.
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Nice work! Thanks for sharing. Looks like you have some good possibilities there.
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What I've learned - don't call a lazy susan a lazy susan to some Susans because Susan will get angry and make a huge scene in your booth.
@Pres, butter trick with teapots - some potters will put a very thin smear of a fat or clear silicone under the lip of the teapot which makes that area un-wettable therefore the spout won't drip. It's a cheat as silicone will discolour with the tannins from the tea and fats will wash off.
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7 hours ago, Simon Heath said:
is the only way to fire to cone 5 1/2 to manually enter the target temperature? In my current firing cycle, I program peak temperature as cone 6 (and yes, it definitely looks like cone 7, though that is the top shelf of the kiln).
You could set it cone 5 then add a soak to get to cone 5 1/2. If you are currently firing to the preprogrammed cone 6 fire and it's bringing cone 7 down then you might want to do either a cone or thermocouple offset. How many thermocouples does the kiln have and have you put cone packs in the bottom, middle and top with the same results?
It's a bit of trial and error fine tuning a kiln firing, there isn't a one size fits all kilns schedule for what works best for all glazes. For this glaze I would be aiming for a cool cone 6, so approx cone 5 1/2 - 6. If your current schedule is going at 108 to 2230 plus you are doing a 10 minute soak I'm guessing (and it really is just a guess) that if you leave the offsets for now and go to 2185F then the 10 minute soak then the rest of the program it would be somewhere to start. I'ld also look at why the kiln can't freefall at 9999 while dropping the 100F for the soak at 2100F and the 20 minute soak there. (I would drop the temp of the second soak to 100F below top temperature of 2185 to 2085)
@neilestrick, I haven't had problems with dropping at 9999F with the Genesis, have you come across this?
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I would call those blisters inside the bowl more than pinholes. There are a myriad of possibilities for blisters, link here with explanations for many of them. Problem comes from figuring out which explanation makes sense. I'm still thinking the problem is in the firing, looks like cone 7 is down most of the way. I have seen this glaze fired to cone 5 1/2 so perhaps a simple test would be to fire lower and see if that solves the problem. I would still do the soak and drop and hold though I would drop at 9999 (rather than 300F) to 100F below your top temperature then do the hold then slow cool. At the same time I would include a fired pot and see if the blisters get worse on refiring. If you do this then put the pot on a waster slab / cookie to catch glaze runs. If blisters get worse on refiring then it supports the theory of a highly fluid yet viscous glaze melt. Adjusting the drop and hold firing in conjunction with the top firing cone would be how to fix it.
Hansen and Britt did some work with this base glaze, links to their articles here and here. I'm not seeing others reference problems with blisters but they do mention dimples (semi healed pinholes) and clarity of the base glaze.
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1 hour ago, Simon Heath said:
But is the fix to just add in the Manganese carbonate, more EPK and silica, or does the effect of the magnesium carbonate (I've become convinced that's what I did) need to be offset?
I think the firing might be at the root of the problem, could be overfired. Could you post a picture of the pinholes and an image of how much running you are getting, plus a witness cone or two.
Adding the manganese will help with the colour correction. Manganese often lends a purple(ish)brown tone to glazes. EPK and silica will bring the alumina and silica up to where they were in the original formula, would have a minor effect on reducing running but probably negligible.
Have a look at the charts below, specifically at the alumina (red box) and silica (blue box). I was simply balancing those by adding the epk and silica. You can see that adding magnesium carbonate has very slightly decreased the COE (coefficient of expansion) and increased the LOI (loss on ignition). Both these are fairly minor amounts and I doubt they contributed significantly to your pinhole issue.
I would just try altering a small amount of the base before trying to correct the entire bucket.
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Couple thoughts,
Do you know how much water you added? Just for the sake of explaining my process here I'm going to just use a figure of 80 water per 100 grams of dry base, adjust it to what amount of water you used, use Brongniart's calculator if you aren't sure. (not including colourants) I would take 180 grams of wet glaze add the 1.8 manganese carbonate plus 2 silica plus 2 epk and try that to get the colour right. For the pinholes, does these firing schedules work for your other glazes and do you verify cone 6 with large cones? This glaze is high in fluxes (relative to the silica and alumina amounts), sometimes pinholes and blisters are from overfiring a very fluid (highly fluxed) glaze with high surface. (zinc contributes to surface tension)
BTW are you sure it's manganese carbonate and not manganese dioxide the recipe calls for? I've seen this recipe with both, doubt it's part of the problem though.
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If you have it on hand already just use regular motor oil, there are other options but this is simple and should work. If you or one of the other members of your group have an oil can to apply it with use that so it doesn't get everywhere.
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18 hours ago, Alyssa said:
(machinery lubricant, not WD-40)
Step one, it needs to be an oil, like a light motor oil to start with. Diagram below, I would oil all the points indicated with red arrows, including every link of the chain. You are correct in not using WD40 however it would help the oil penetrate if you oil first then use WD40 to help the oil penetrate.
Diagram below, oil all the areas indicated by red arrows. Blue arrow points to a slot that needs to be cleaned of any clay debris. Only other thing I would look at is where the thread goes into the block, make sure that is clear/clean.
Step two, if necessary would be to unhook the chain from the sprockets and see which side binds, it might just be the chain itself.
I have the same slabroller, the turning wheel started to bind and needed to be oiled which fixed it. The bearing surfaces seem to seize/gall more than expected.
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1 hour ago, Simon Heath said:
I don't know what I used - fluffy white powder (ha!), maybe Lithium carbonate, but who knows.
Most common mixup with manganese is to use magnesium instead, magnesium carbonate is very light and fluffy so it could be this. Either way, lithium or magnesium carbonate will lower the expansion of the glaze. Did the glaze run excessively?
I wouldn't toss the glaze just yet. Pinholes are often a firing issue, either inadequate burnout during bisque or the glaze firing schedule needs tweaking. I would suggest posting your clay type, bisque firing schedule and glaze firing schedule and glaze recipe before getting rid of the glaze.
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Hi Alyssa and welcome to the forum!
Have you tried adjusting the tension on the cranks? pdf here if you need it on doing this.
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9 hours ago, nancylee said:
I don't want to throw one, as those looked complicated and finicky and I like to handbuild more anyway
IMHO if you already have a moderate level of throwing skill I think it would be much easier to throw than handbuild. If you do try and throw one keep your walls from getting too thin at the top where they will be curved into each other.
Large amount of iron oxide in slip
in Clay and Glaze Chemistry
Posted
I have an iron wash recipe from Bob Kingsmill, image of one of his masks below with it. It doesn't run as is at cone 10 reduction but if you add a small amount of gerstley borate it does.
Kingsmill wash (I've used it from ^6 - ^10 ox and reduction). This recipe is from about 25 years ago, he might have changed it since.
One of his masks with the iron wash