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Posts posted by Min

  1. @Katie Piro, I've tested the High Calcium Semi Matte 1 base with 12 zircopax plus 2.5 tin oxide. It's a lovely white and still a semi matte with these additions. It does bump up the silica level (from the zircopax) but I found it still matte enough for me. I was using it over a white clay, I don't know what colour your clay is but will need higher opacifier levels over darker firing claybodies. I found it did slightly cutlery mark. If you also find it cutlery marks then do a line blend of the semi matte glaze plus a clear glaze, both with the same levels of opacifiers. There should be a point where the cutlery marking stops without the glaze becoming too glossy.

  2. 4 hours ago, CactusPots said:

    But does that get you a dead sharp line between the glazes?

    If the 2 glazes are similar in composition there is going to be less chance of them flowing together and creating an uneven line during the melt. If you want a dead sharp line then avoid having one glaze high in alumina and the other low in it (for example), what the one glaze is low of it will pull from the other glaze. If you use a gloss over a matte or vice versa there is often more visual texture happening between the two glazes than having the same (or very similar) bases (plus colourants or opacifiers) overlapping. Same principle for keeping a sharp line between glazes. Other thing is to make gravity work in your favour, do the change between inside and outside glaze at the highest point on the rim so any flowing that happens when they melt doesn't mingle with the other glaze.

  3. 40 minutes ago, JohnnyK said:

    Why not make the above out of clay? Throw a shallow bowl with a hollow riser in the middle and trim a foot ring in the bottom with a notch or two or three (like a berry bowl). Open the hollow shaft to the bottom and scribe a glaze level line on the inside...

    Yup, that would work too! Maybe a little faster to make using a cake pan, doesn't need to be glazed and fired. But my version would need to sit on something.

  4. An idea I've had for years but never actually got around to making is a glaze pan for doing rims of mugs etc. Glaze the outside like Liam said then to get an even line around the rim dip in a pan with a threaded shaft through the middle to let the air escape. Marker line around the inside of the pan (black line) to keep the glaze level topped up, hole drilled through the pan (red circle) and threaded shaft (blue) with a washer and nut on each side to keep it snug, probably need a little gasket to stop leaks. Would have to sit it on a container of some sort since the thread goes out the underside. Think this would prevent glaze burps inside mugs etc and make an even glaze line.


  5. If there is any chance of making your own casting slip instead of trying to alter a commercial one I would go that route. Claybody colour would be far less expensive to achieve by using darker firing clays. Manganese speckle might or might not stay suspended long enough in the casting slip to work, I'm not confident that the larger bits would, also the larger manganese particles probably wouldn't be on the outer surface of the cast so I'm not sure if they would show. An example of a similar colour casting slip here, the M340 revison 7 titled L3798G. It looks like it needs to go to cone 7 to get the absorption under 2% for functional work. (or it might tighten up enough with approx 1point 5 talc added for cone 6) 

  6. 5 hours ago, larathompson288 said:

    I have two types of clay and I was wondering what the best firing schedule is for them for bisque?

    This can vary depending on how thick the pieces are but in general a slow bisque firing would be the safest until you get to know your kiln and firings. Temperature varies depending on how fast or slow you fire the kiln. Turning the kiln up quickly will take a higher final temperature to reach a specific cone rather than going very slowly to reach the same cone. In effect the cones are measuring both the time and the heat spent working on the clay. Temperature measures just that alone. 

    Cone 04 (approx 1060C) is a common cone to fire to for functional work. It leaves the bisque strong enough to withstand being picked up with tongs and yet still porous enough to take up the glaze. Darker clay bodies can benefit from a slow rise in temperature through specific ranges. More on that here if you need it.

    All that being said a cone 04 (approx 1060C) slow schedule for bone dry pots and using a programable controller below, this will take approx 13 hours.

    - 26C hour up to 120C  no hold

    - 90C hour up to 540C no hold

    - 38C hour up to 590C no hold

    - 80C hour up to 900C no hold

    -27C hour up to 1060C no hold

    If your pots are not bone dry then add a "candling" at the beginning. This is just holding at 90C for how ever long it takes to finish drying the pots. This schedule can be sped up but I'ld suggest trying this slow one first and see how your clay and glazes turn out. 





  7. I compared your recipe to quite a few that I've tested, including one from Andrew Martin. (he has a great book on slip casting if you need one) Have a look at the formulas (below the recipes), I'm not seeing anything thats really out of whack. If it casts okay I'ld be looking at other causes for the chipping and crazing. For both I would confirm that the kiln is firing to a full cone 6, verify with witness cones on each shelf.

     Depending on what qualities of the glaze is contributing to the type of matte glaze it is makes a difference to the scratch resistance of the fired glaze. Cutlery marking could  just be from underfiring also. For interior surfaces of functional pots a gloss glaze is (usually) preferred for scratch/cutlery marking resistance.

    It's hard to tell in your picture but it looks like the rim on one of the black pots is quite thin plus the edges look like they could be rounded over more. Sharper angles on rims tend to chip more than rounded rims.


  8. Kiln bricks to carve out plugs make really good plugs but it's dusty if you use power tools, I've found whittling them by hand isn't as dusty but still needs to be done outside (wearing a respirator).

    Other option would be to use peep "flaps" instead, 2 of my kilns use these. On the Euclids kiln they are made from an open groggy clay, on the ConeArt they are stainless steel. Would need to add 2 self-tapping screws for each. One screw to attach and pivot the other screw to hold it in the open position. Advantage to using flaps instead of plugs is they don't get dropped or lost. Disadvantage is they do get hot so can't touch them with bare hands but easy to flip them over with a scrap of brick or a post (or wear gloves).

    edit: let me know if you want pictures and I'll take some closeups. 

  9. 1 hour ago, njsceramics said:

    Is it ok to cover first in a black underglaze and then apply? Should I underglaze then fire then apply Dry Lake then fire again?

    Underglazes contain binders that when dry leave them somewhat resistant to accidentally being rubbed off. These binders can inhibit the absorption of subsequent glaze layers. Often this isn't an issue but crawl glazes are notoriously difficult to adhere to the pot so yes I would recommend re bisque firing the underglaze on prior to glazing with the Dry Lake crawl glaze. Just add them to a regular bisque load or if you have a small kiln (or a lot of underglazed pots) just heat them to around cone 022 (zero twenty two) or approx 1000F / dull red heat, this is hot enough to burn off the binders.


    1 hour ago, njsceramics said:

    What would be a good underglaze for bisque fired pieces?

    I've used Crysanthos, Spectrum, Speedball, Amaco LUG and Velvets, Seattle Pottery Supply underglazes plus some homemade ones. They all worked on bisque, Speedball is the least expensive. Everyone of them except the Seattle Pottery Supply one needed to be watered down before using it, if you lay down too  thick a layer it can lift during the firing. If you want opaque coverage its better to apply three thin coats rather than one heavy, leave each coat to dry completely before applying the next coat.

    I strongly suggest doing some test pots before doing your "real" work, crawl glazes can be tricky. Application method shows and thickness makes a huge difference in the results. 

    Welcome to the forum.

  10. If you have an accurate set of scales I'ld add an opacifier to the clear instead of mixing it with the white glaze. I'ld suggest weighing up 200 grams of really well mixed glaze and add 14 zircopax to it. Dip a test tile just how you would dip a pot. Now add another 4 zircopax and dip another test tile. Add a further 4 grams of zircopax and dip a third tile. Repeat a fourth time by adding a further 4 grams of zircopax and dipping another test tile. Fire the test tiles and go from those results. If you don't like the look of the zircopax opacified white then consider using tin but use half the amounts for your tests. (tin is far more expensive than zircopax though)

    Could work to mix the clear with the white glazes but in effect what you would be doing is diluting the strength of the opacifier in the white glaze by mixing it with a clear.

  11. Mishima technique is good for fine lines. Scratch the fine lines into leather hard clay then flood the incised lines with underglaze or slip. When the u.g. or slip has dried scrape it back to leave it in the lines, a flexible metal rib works well for this. Other way you can do it is to apply a water based wax resist to the leather hard clay then scratch the lines through and flood with underglaze. Let it set up then sponge off excess from wax. (Forbes wax works well for this method)

  12. 4 hours ago, Harriet9 said:

    I haven’t been adding barium carbonate as this is the first time this has happened in 2 years of no issues.  Will go back to my old recipe and see if it does the same, if so will try bc. Tiny amounts right?

    I'ld try 0.30% of the dry weight of materials. (thats zero point three), if this isn't enough then bump it up to 0.35%  (zero point three five) Don't eyeball or guesstimate this. 

    4 hours ago, Harriet9 said:

    There was also more warping which I’m thinking may be the extra water. 

    I think what happens when the specific gravity is too low (ie an excess of water) is the walls not only take longer to cast but they won't be as dense. This could very well lead to more warpage, this also could be why they feel rougher than previously. It's also going to age your moulds faster and increase the drying shrinkage of your pots.

    I would really suggest getting your specific gravity dialled in. 


  13. A few thoughts in no particular order - 

    Do you measure the specific gravity and viscosity of the slip? Adding excess water is obviously going to mess up the specific gravity so I'ld let some evaporate off until the specific gravity gets back to what it should be.

    If you are mixing the slip body yourself are you using the same bags of materials or has one of them changed? Same stains? Any chance of an error with the weighing of the last batch?

    Have you verified you are reaching your target cone with witness cones? 

    3 hours ago, Harriet9 said:

    to help improve the vitrification, this is a vitrified earthenware body

    I know in England there are some earthenware bodies that actually fire up to cone 6 and what we would call midrange in North America, typically we don't see vitrification of earthenware bodies though. Mature yes but not vitrified. Semantics perhaps. What is the absorption percentage of your body?



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