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Min
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Posts posted by Min
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Hi and welcome to the forum!
Could you give a bit more info about the underglaze you used? Firing range of it? Glaze info?
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18 hours ago, Franny said:
The first is how to control the movement/blurring of the oxide decoration. I've come across suggestions of adding gum arabic or glycerine to the oxide mix to 'fix' the decoration in place, but suspect this will simply burn out in the glaze firing and the oxide decoration will still move as the glaze melts. I suspect that controlling movement and blurring of the oxide decoration may be more a matter of finding the sweet spot of glaze melt-fluidity, and this may involve increasing the alumina to silica ratio in the glazes used over the oxide decoration. Am I on the right track?
I would agree. Fluid glaze vs stiff glaze will make a difference as to the colouring oxide movement. If you think of a majolica glaze for example, they are very stiff glazes with very little movement. Another factor will be which oxides you are using, copper for example will be very difficult to keep with a crisp edge, stiff glaze or not. If you find just using china clay + colouring oxides + water/matcha doesn't give you the effect you are looking for you could try underglazes, either in prepared form or powdered that you add your own medium to.
18 hours ago, Franny said:The second issue concerns how to ensure that the oxide decoration is fully 'encapsulated' in the glaze to ensure, particularly with toxic oxides such as cobalt, that any potential leaching is minimised.
I think common sense plays into this aspect insofar as brushwork over or under a glaze and the quantity of the colouring oxide present. Only way to know for sure how much leaching is happening would be to have a sample lab tested. Before doing that I would do a basic acid and base leach testing yourself, this will rule out ones that are definitely leaching, they could still be leaching a small amount that isn't visible to the naked eye. (Home tests rule out glazes, don't necessarily rule in glazes)
Welcome to the forum!
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If you replace the gerstley with a frit then you have a few things that like to sink to the bottom of the bucket, nepheline syenite being a big one plus the frit plus the silica. 10 epk might not be enough to suspend them.
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4 hours ago, Rae Reich said:
@Min, do you throw standing up? What’s that tall thing that looks like a shifter beside the wheel pedal?
Yup. I missed driving a stick so I added one to my wheel's foot pedal.
Piece of redi rod attached to the side of the pedal with a small door knob threaded to the top.
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I'ld replace the gerstley borate with a boron frit to supply the boron as gerstley is notorious for gelling glazes. Given that this glaze has only 10 clay you might then need to add 2 bentonite to it given it has a large amount of nepheline syenite and the frit. Magnesium carb can actually flocculate a glaze but given there is only 3% that might not have this effect. How are you measuring your specific gravity? Is your scale tared out and accurate?
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Throwing area and slab roller, shelving to the right of the wheel, large drying cupboard on the left beside the pugger. Glazing and kilns in another area.
- GEP, Kelly in AK, Hulk and 5 others
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There is 2 hours difference between the two, is it worth the risk for deadline work to try it now if you 100% need the work not to have issues? Step 4 and 7 are an hour faster with a fast bisque. I only use fast bisque (different kiln, same schedule) with white and porcelain clays with no issues. It's the speckled and dark clay that if you are going to have issues with that might be of concern.
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45 minutes ago, Pir said:
Is Insight better software? Is it open to use?
Whichever one you are comfortable with is the one I would use. Insight isn't free.
46 minutes ago, Pir said:Did you have any idea about why the Glazy quantities looked the same but the ratios looked different?
It's just fine tuning the math, round up or down by the hundredths and you can get the ratios exact. I don't know to which decimal point Glazy will go to. Probably not significant.
Yours is the first in the screenshot below, fine tuned in the second to have the ratios exact and supplying the magnesium from magnesium carb, you can compare it to your altered recipe and see the tiny differences. Data into the materials database could alter slightly between software, in most cases it won't be significant. Extra 10 silica like in the original Britt recipe would likely gloss up the glaze somewhat. Notice the LOI has increased from 7.7 to 10.9, likely because of the magnesium carb.
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Callie has the recipe above with silica at a total of 20 (10 325 mesh + 10 200 mesh), your original recipe has the silica at 10 plus custer instead of G200 which ones do you want me to use?
edit: I think this is why my silica level was higher
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47 minutes ago, Pir said:
Min, I use Glazy--is Insight different? I see on Glazy that while the two formulas show identical quantities of oxides, the ratios are off. That's always confused me.
My bad, I had a typo entering the recipe.
Post a screenshot of the recipes so we can see the ratios.
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@Pir, I'ld get rid of the bentonite with that recipe to start with, once you get the specific gravity at a level that gives a good glaze coverage and it's still too thick then the next step I would do would be to calcine most of the epk, I'm not seeing this as a recipe that would need a deflocculant at all.
I just plunked your altered recipe into Insight, the silica is a fair bit lower in your version, need to bump it up from 16 to 24 then retotalled to 100
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I would suggest contacting New Mexico Clay and asking them about their micaceous clay, let them know you are looking into slip casting and if they have any info or experience with this as not all clays cast well. If they don't know I would ask if you could purchase some of their dry lowfire mica clay and start testing it. Can't speak to the paperclay addition as I haven't cast with it but I have read articles of people using fibres in the slip, my hunch is you wouldn't need it.
Sumi Von Dassow's book "The Potters Kitchen" would be a good resource for shapes, process etc.
19 hours ago, Ryan M Miller said:it means that I can make a stackable set of matching cooking pots more easily and faster than if I were to hand-build the pots or throw them on a wheel.
How many pots are you talking about here, selling them commercially or just enough for your own use?
I'ld also make sure you have a really good insurance policy if these will be sold.
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By cooking pots do you mean pots for the oven or a gas stovetop?
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I don't think we can say if this would work or not, too many variables to say for certain. Clay type will make a significant variable, for example if it's a dark burning clay chances are it would need a clean bisque firing to have all the organics burn out without causing pinholes or other issues with a single firing. We also don't know if the clay plus engobe plus glaze are all compatible with each other, in other words do the engobe and glaze both fit each other and the claybody without crazing or shivering? Some glazes work better for single firing than others, as do some clays. I'ld run some test tiles before trying this on "real" pots.
Welcome to the forum!
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@Zeynep Gedikoglu Sculpture, are these lowfire? Wondering if you could insert a really thick bead rod into a hollow cavity as far as possible then have it embedded into a large lump of bisque fired clay with a snug hole that a few inches of the rod would fit into? How heavy is the piece? Thick walls?
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Hi Ryan and welcome to the forum!
If you can't find a specific topic you are looking for feel free to post a question in any of the sections.
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If the unglazed bottom is really vitrified it should be fine but if not I would seal it before soaping.
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2 hours ago, Callie Beller Diesel said:
My working theory on why you get so many different instructions about slip/score/water/no water is that it varies by clay body.
+1 for this!
I think it goes far beyond attaching handles and that is perhaps why it takes so long to get processes nailed down when starting out. Ask 5 different potters how they throw / slab build / dry / make attachments / which batts work best / bisque fire/ glaze fit / glaze firing and so forth and you could very likely get at least 5 different answers for each process. Type of clay, working environment (humidity etc), work style, clay thickness, material chemistry etc all come into play. Definitely not a one size fits all scenario for much of what we do.
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5 hours ago, Tina01 said:
I mixed about 20% sea sand to it
I wouldn't do this until you have fired some test tiles to see what the sea sand is doing to the clay. If it contains calcium (from seashells) it could really affect the firing. Also 20% sand is a lot for making small(ish) pots like in your example. I'ld suggest starting from the beginning and making some tests with 5%, 10% and 15% sand. Wash the sand, dry it then weigh out the amounts and add it do dry clay and slake it down, mix it up with a paint mixer then when workable make some test tiles and fire those (with no glaze) to whatever cone the clay is rated for then examine the test tiles.
Was it sea sand that was recommended to you or just "sand"? In ceramics when sand is mentioned it's silica sand. Silica sand comes in different mesh sizes, the smaller the number the courser the sand.
I'ld also try making some pots without any added sand, clay might be just fine the way it is.
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23 hours ago, oldlady said:
min, that 3 point lift gadget looks perfect! do you have a brand name or source for purchase? and your squishy bucket idea is terrific, too.
I bought the plate lifter from Aliexpress but they are on the river in South America too and I'm sure lots of other places too. Just google steamer plate lifter. If you have a lot of 4 sided pots or square tiles then maybe the second one in that link might work better?
If you make a squishy bucket what I did was to heat up the two areas of the bucket (that would become the end points) with the heat gun to get it to squishable, one end at a time then put a board across it and some weights on top to hold it squashed while it cooled down. This was after I cut the bottom off.
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Glazing pots today, couple pictures of things I use that make it easier for dipping and cleaning up plates or other flat(ish) pots.
First one is a tool for lifting plates out of a steamer, I use it for dipping flat things in glaze, doesn't leave any marks on the topside of the pots to clean up. 3 small contact points underneath, my glazes are fairly forgiving so I don't bother wiping them out. It's also good for when you only have a bit of glaze left in the bucket since you dip horizontally and not vertically. The one I have opens to about 12".
Other thing I use is a 5 gallon bucket with the bottom cut off with a jigsaw then squeezed into an oval for pots that are too wide for the buckets or bowls. I line it with 2 kitchen bin bags and tape the outside. First time you use it the air between the bucket walls and the bags will puff inwards so you need to make a tiny incision in the plastic bags near the top then the glaze will push the air out as you fill it up. I put it in a rectangular bin while using it just in case it springs a leak.
- Kelly in AK, Hulk, Pres and 2 others
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Quick Glazy search brought up this one. Don't know what adding a glaze on top of the decal would do. Might be faster in the long run to remake the piece and start again.
Problem with royal blue underglaze (cobalt)
in Clay and Glaze Chemistry
Posted
If you have a look at the underglaze it says "For best colour response at 1280 we recommend low or no Zinc stoneware glazes." Then have a look at the glaze you are using, it contains zinc. "Midfire to Stoneware Clear Glaze containing approx 5% of Zinc and 5% Calcium" Even though you fired to 1250C and not the full 1280C this combined with what may be a heavy application would be my hunch as to why you got the colour response you did.
Which porcelain glaze did you use? Have a look at what the writeup for it, see if contained zinc, you can also look at the Safety Data Sheet (SDS) for it if it isn't clear.
For your second question, for functional wares you want to fire the clay to it's maturity to make pots that are as vitrified and strong as possible. I don't know what the maturity for your clay is so I can't answer that one for sure.
I know it seems very wasteful to fire a kiln with just test tiles but this is what I would do rather than taking a chance on a kiln load of pots. Ask your supplier if they can recommend a zinc free clear for your clay and underglaze and make some test tiles. Try the underglaze with 1,2 and 3 coats then glaze with a thin, medium and heavy application and see what happens. Sometimes the unexpected happens and we don't care for it because it wasn't what we were going for but I think the purple areas look nice on your pot.