douglas got a reaction from Babs in Teapots cracking just above foot when filled with boiling water.
One other possible cause is too much glaze or using a runny glaze on the inside of the pot. Maybe when you are pouring the glaze out it is taking a while and the inside has a thicker than normal application? If the glaze is too thick at the bottom of the pot, and if the glaze doesn't shrink at the same rate as your clay body during firing, the thick glaze pooled at the foot is creating a stress crack and the sudden temperature change is causing it to fracture along the spot where the thick glaze ends and the clay wall begins.
douglas reacted to Marcia Selsor in First porcelain fire: questions
I add alumina hydrate to my wax resist for bottoms of porcelain AND flanges on lids. I put about a cup in a jar and stir in about a tablespoon of alumina hydrate. Porcelain can flux enough to "pluck" or stick to shelves and where bare clay touches bare clay as in flanges. Alumina in the wax prevents that.
douglas got a reaction from Min in 2 piece bowl mold help!!!
If nothing comes out when you flip the mold over, then you might not be filling the mold up completely. If the walls are hollow that would explain why you are getting two halves instead of a solid bowl.
You need to screen or thoroughly mix the slip to remove the lumps. You may also need more water in your slip if it is not able to run all the way into the mold before it dries at the base, blocking you from adding enough slip to fill the mold.
douglas got a reaction from Chilly in Confused about Crazing
Glaze fit means the glaze shrinks at a rate close enough to the clay body shrinkage, that it will not crack or shiver off the clay. Just because your glaze and clay are firing to the same cone, does not mean the glaze will fit the clay. Apologies if you know this, but it seemed like you are equating firing temperatures with whether the glaze should fit in your descriptions.
The root of the problem you are experiencing is you are buying off-the-shelf products. Since you don't control the ingredients you don't have control over whether they play well together in the kiln. Learning glaze chemistry is an option, mixing your own, and adjusting to fit your clay body, but that takes most people a lot of time to work out. If you want to be able to just buy glazes and clay, then you should ask your vendors to recommend clay bodies or families of glazes that should work well. If your glaze vendor's response to using earthenware was don't use earthenware, then you either need to follow their advice and choose a different clay body, or keep your clay body and find a different glaze.
douglas reacted to neilestrick in Soft Brick Repair
If I'm understanding the direction of the hole, it won't show when the bricks are used for a kiln wall, correct? I wouldn't even worry about the hole. At 5/8" it's likely not going to affect the insulating properties of the bricks enough to matter. If the hole will show and allow heat to escape, then fiber is probably the best way to go. Wear a respirator when using the fiber.
douglas reacted to PSC in Clay Thickness Before It Explodes?
Its not the thickness but the time allowed to preheat and fire...thicker ware needs a longer preheat and low cycle. The key is to get all the moisture out of the clay before the molecules start firm up in the kiln. The moisture needs to escape before the passages start to close. An over night preheat and a long low cycle could insure the survival.
douglas reacted to Min in removing large platter from the wheel
@hantremmer, another option is to leave the wheelhead as it is and use a clay donut or pancake to stick the batts to. Use some scrap clay and throw and flat, level donut or pancake on the wheelhead then put your batt on that. For plaster batts and wooden ones under 8" or so in diameter I use the Xiem BatMate, works really well, the batts need to be flat and not warped, no pins necessary.
douglas reacted to Min in firing a fitted lid
Hi Chantal and welcome to the forum. Just to add a little to what Liam posted, 1-2 tsp of alumina hydrate to a cup of wax resist plus I would add some food colouring so this resist is a different colour than your regular resist. The alumina hydrate will want to settle out of the resist so stir it often while using. Also, when you use it be very careful not to get any of it on your glaze as it will fire to a very rough texture.
edit: with your gallery, if you make the inner part of the flange at a slight inward taper, not the right angle like in your image it makes the lid fit easier.
douglas reacted to neilestrick in Handle size for mugs ?
Like Liam, I make the top attachment of the handle to be approximately 1/3 of the width of the mug. the handle thickness should be a similar thickness to the lip of the mug, so they carry the same visual weight. The handle should taper quickly so it springs off the mug in a natural arc. Assuming it's intended for 2 or more fingers, I like a 'D' shaped handle. I'm kind of picky about handle shapes- I hate '7' shaped handles, or handles that loop up above the rim before going down. Currently I only make 1 finger handles. I like how they feel, and they fit the style of my mugs. Some people really hate 1 finger handles, and aren't shy about telling me when they come into my booth at art fairs.
douglas got a reaction from Rae Reich in Cracking in 20 inch greenware platters
My guess is you are letting them dry on plastic or some other smooth surface. The rim might have adhered to the surface (lots of moisture trapped under plastic could make a small amount of slip where the rim touched the surface). When the foot and insides of the platter shrank, the rim stayed put and caused the cracking.
One fix for this is to use paper or foam under the pot while it dries so the clay can easily move on the surface as it shrinks.
douglas got a reaction from Rae Reich in Faceting tools
Use a cheese cutter wire with adjustable roller. Hold your thumb on the roller while you cut or glue it so that it won't roll. Hold the wire against the rim of the pot, and adjust the depth of the wire to the roller to 1/3 or 1/2 the width of the clay wall.
Then when you cut the roller will prevent your from cutting too deep (assuming consistent clay wall thickness).
douglas reacted to glazenerd in Understanding COE
Coefficient of expansion (COE) is a mathematic equation that measures compression and contraction. The clay body is compressed by the contraction of the cooling glaze. Pure silica glass has a COE of 5.5 x 10(-7) power: glaze calculators just hide the 10(-7) and only show the 5.5. When you add feldspars, alumina, and other oxides: the COE rises.
Clay and glaze both have the basic elements required to make glass (glaze): silica + alumina + fluxes = glass. The difference between them is particle size, ratios of each, and flux levels. Flux levels in clay are much lower than flux levels in glaze. That difference in flux levels is the primary reason there are appreciable differences in clay and glaze.
To understand why COE varies so widely: you need to understand how each glaze ingredient raises or lowers COE. Here is a short list of common glaze ingredients and their individual COE values:
Nep Sy: 9.86 Mahavir potash 8.60. Silica. 3.52. Alumina: 6.47 EPK. 5.06
Any given glaze recipe has a given % of flux, silica, and alumina. The list of ingredients simply supply various amounts of each until a given level of each is reached. There are minimum amounts of each that are required to achieve a complete melt at any given cone: these minimums are known as formula limits.
**** see " Hesselberth & Roy" for formula limits.
To paint a simple picture of glaze formulation: as the levels of fluxes rise ( sodium, potassium, boron) the Final COE rises. As silica and alumina increases; the final COE lowers. The reason EPK is commonly suggested to fix crazing ( cracking) in the glaze is because kaolin is 37% alumina and 48% silica: both lower COE while maintaining formula limits. The other simple premise: the lower the cone fire; the higher the level of fluxes required to achieve melt. As the cone fire gets higher: then flux levels are lowered: which in turn lowers COE. Cone 6 recipes typically run 50-55% fluxes in the recipe: more means lower cone fire, less means higher cone fire. This general rule will help you figure out if you have a low, medium, or high fire glaze recipe. Obviously there is wiggle room at any given cone range.
The COE values of cone 6-10 clay bodies run 5.5 up to 6.10 typically. There are variances for highly translucent porcelains, and high talc content low fire recipes. It is best to ask your supplier for the COE of your clay of choice. Glazes typically run 6.95 all the way up to 9.00 (crystalline glaze). Normally they are in the 7.25 to 7.95 range. The trick is to keep the COE of your glaze within 1 to 1.25 of your clay.
Example: clay COE of 5.75 plus 1.25= 7.00 for glaze COE. This is a target range, not an exact number.
The above is the chemistry and science between clay and glaze COE. However, let's throw that in the trash and use something easier. No, this is not accurate but it conveys COE differentials between clay and glaze in digestible terms. Look at COE in terms of percentages.
A clay body with a COE of 6.00 will shrink 6.00% from peak temperature to room temperature. A glaze with a COE of 9.00 will shrink 9.00% from peak to room temperature. The clay is shrinking at 6.00% and the glaze at 9.00%: that is a 50% difference in shrinkage rates. As the differences in shrinkage rates climb: crazing issues increase. The closer you can get these two values together, the chances of glaze issues goes away. The terms are compression and contraction: but shrinkage conveys both in easy terms. When the clay and glaze are within acceptable percentages of each other: the term is "glaze fit."
Hope this helps.. Tom
douglas reacted to LeeU in How to start a path toward making pottery a career?
Whether as a full time, income generating, business or a satisfying low income job or part time avocation, know up front it will be expensive. I took small business classes as I earned my BFA in ceramics and was able to plan out a "how to, and how much" if I were to pursue clay work as a career. I did not go that route and only returned to it a few years ago, establising a home studio after I retired. Knowing the cost projections was very helpful--if I went in blind about the start-up and ongoing expense I would have been sorely disappointed at how long it could take for any investment to begin to pay off.
With issues such as depression, or any health challenge, it is important to factor in the wear and tear, the known cycles, the possible practical limitations, and the obstacles any such condition might pose when trying to ignite enough fire to sustain interest and push through over the long term. It is easy to compare oneself to others who seem to be having an easier time of it and misjudge the reasons as having to do with talent or motivation (i.e. self-blame or lack of encouragement from others) rather than confronting the reality of the fallout from a serious health condition. I had an instructor who chastized me when I disclosed I was having health problems that were affecting my work but that the work was keeping me going. He told me "art isn't therapy" and suggested I should quit. I was almost crushed, but my nature is to scrape it off my shoe and say "Oh yeah, watch me", so I came through OK. Lesson learned, support from people who understand such dynamics is crucial for channeling my passion into a steady state that is at balance with the rest of my life.
Becoming a professional potter is no different than establishing any other career--requires hard work, time, money, ability to withstand set-backs, and above all the willingness to learn the tools of the trade (the chemistry, the techniques, the history etc.) Take the best, most comprehensive courses/workshops you can find. Also check to see if there is a local or regional Potters Guild where you are and join it.
douglas reacted to Min in Help with 3rd/4th Glaze test results
Cones in the "Fast speed, no hold" are at a perfect ^6. The tip of the ^6 is level with the top of the base, exactly where it should be for self supporting cones. I think the question is did you have cones on all shelves? What cone did the pot on the bottom shelf get to from that firing? Need that information. As to the fast firing cycle dropping the cones more than the med cycle it could be due to the amount of mass in the kiln, especially in the area around the thermocouple. Just to confirm, your kiln has 1 thermocouple, roughly in the middle of the kiln?
douglas reacted to neilestrick in Lithium replacement
We had a long discussion about copper some time in the last year. It's true that it takes a lot to cause a problem, although there are folks who are medically sensitive to it. If it's obviously leaching, like with a vinegar or lemon test, the glaze should be fixed.
douglas reacted to Mark C. in Salt Kiln Conversion
Today my wife and I fibered the interior
I like to make button gaskets out of delaminated fiber about 1/2 thick
You need to add loft to the job as the fiber will shrink at cone 10 no matter what its says on box-I tend to add this loft in two directions .
I made these buttons and shipped them over.
I use these pliar's to make the pig tails which keep tension on the button.The gaskets keep heat off the wires and add life to job.
douglas reacted to Mark C. in Question on mixing colorant batches
I always do glaze tests starting with dry weights
lets say you want to add cobalt in 5% additions
mix up 100 grams with the starting 5% of cobalt then dip a tile then add another 5% of cobalt -that will equal 10% dip a tile and so forth.That way you are adding the right amount in dry weight to your original batch
You can do the wet thing but I feel there is more room for error in small batches to begin with and wet just adds more variables.
Thats my 2 cents
douglas reacted to JessicaGrayCeramics in Why is some bisque fired to cone 06 and some to 04
Bisque firing you work to 06 is perfectly fine whether it was meant for food use or not. The difference is Some clays become a little more solid at 04. Clays that are mid or high fire may need a slightly higher temperature to mature. However if you are using a clay meant for low fire I'm sure it would be absolutely fine that it is bisqued to 06. If you do wish to re-fire the piece to 04 before glazing the piece, just for piece of mind you can. However I do warn you that extra firings can put stresses on the wares and cause them to crack. I bisque fire to cone 04 on a regular basis with most of my work. If you take a look at the image attached you can see a lot of little protruding tabs that stick out on my work. I use a mid-fire clay and fire to cone 6 for the glaze firing. The way my work is made the tabs break off too easily at cone 06. So I have figured out that if I fire to cone 04 the strength of the tabs on my work is much greater. Bisque firing can also be done at cone 08. The cone necessary really depends on the clay body and the artists techniques. It is a personal thing depending on how you use your work. It is not necessary to bisque to 04 unless your work needs it for stability.
douglas reacted to Min in cone 6 red glazes
Chrome tin reds just need the tiniest bit of chrome, like in the range of 0 point 2 If you are making up a 100 gram test batch you need a well calibrated scale that can accurately do tenths of a gram. Sounds like you just used too much chrome for those tests. Copper reds are fired in reduction to get red, you will get green in an electric kiln.
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