Jeff Longtin

Welcome to the Forum Joel. It sounds like you've been talking to someone who is describing making plaster molds for casting metal or wax. Is this person actually casting clay into their molds? The process also sounds like the process for making "mother molds" for vertical sculptures. Is this person doing that? The main reason we use plaster to make molds, for casting clay, is that it is porous and absorbs the water out of the clay. If we add something to the plaster then we're introducing air pockets into the plaster wall. In theory this could change the way the plaster absorbs water. Rather than add things to the plaster I prefer to simply make a thinner mold. The ideal thickness for a casting slip mold is about 1.5". (per USG) I've made molds that had thicknesses as low as .75" that still worked fine for casting porcelain. (They became saturated, more quickly, but they at least allowed for one casting a day.) Would a thinner mold be an option for you? My main hesitation with the method described is that the artist is creating a thin layer of plaster against the surface of the model. If she doesn't apply the next coat evenly she may be leaving behind large air pockets in the plaster wall. THOSE air pockets may affect absorption greatly.

A common culprit for these kinds of problems is too much water and/or not enough compression. Do you use lots of water during the throwing process? If a pool of water sits in the bottom of the form, as you throw, this can lead to cracking/blowout problems. Compressing the clay, with a rubber kidney tool, is a good way to strengthen the bottom of the form. As Bill suggests overworking a form can also stress the clay. Using a kidney tool shaped like your desired shape can reduce the amount of time your fingers are actually working the clay.

And lastly, an odd method that sometimes works...fill the mold up with slip...wait a few minutes...then empty the mold. Allow the slip to set up a little bit...then repeat. (Essentially emptying the mold before the slip has a chance to gel.) You can only do it a few times, and the casting will likely be thin, but it might allow you to cast the object and have a reasonable chance of it casting uniformly. This only really works with porcelain. (Because it's a fairly dense clay.) It's quite something to cast an object that's eggshell thin and have it fire out.

fyi: a teardrop shape is probably one of the most difficult shapes to cast. You're not alone in having this problem. Another suggestion: stick a small stick into the slip, every 5-10 minutes, and stir the slip around, gently. This way the slip will not set, entirely, and it should pour out more easily.

Good Morning Kath_of_Khan. Welcome to the Forum. Yes, adding more water, to the slip, is the solution. The slip may be too heavy in deflocculant but that can only be affected if you're mixing your own slip. Another solution is to wet the mold before you pour. (The inside of the mold.) This will slow down the speed at which the slip sets. good luck

Good advice so far...I'll just weigh in with this: one big difference between a soft bisque firing and a normal one is that the pots are more fragile. If you use glaze tongs you need to be especially careful not to grip the pot too hard. Super easy to go thru the clay wall or shatter the piece entirely. Tongue method - in a pinch it works however I prefer to lick my finger then touch the pot. Bisque pots always seem to have a slight dusty/salty surface that seems a little unpleasant. As mentioned, an underfired pot will absorb more glaze than a properly fired one. Sometimes wetting the pot with water, a gentle sponging, will counter the very dry surface.

Good Morning Eve, Welcome to the Forum. Do you have experience making molds? You can make a new mold piece but since your greenware piece has already shrunk a little bit I would make a complete new mold instead. To be on the safe side I would also fire the greenware piece to bisque temp. Then seal it with acrylic paint. To simplify your life I would use the remaining mold piece as a starting point and use it in the molding process. Making sure the bisque is sealed and applying three coats of Murphy's Oil soap, to the plaster mold piece, pour a new mold side. The plaster will probably seep down between the mold and the bisque piece but that's alright. After the plaster has set up, 30-40 minutes, open up the mold and clean the seam. Apply three more coats of Murphy's, to the new plaster mold piece, and pour side 2. If you were hoping to just drop the greenware piece into the existing plaster mold and pour a new plaster piece you could do that BUT you run the risk of the plaster not cooperating and going places you didn't expect. Then you may be forced to clean new plaster off an old mold. (Been there, done that...not fun.) Good Luck

Good Morning Elaine. I'm sorry this happened to you. Very frustrating! Bad firings always occur right before big events. I'm in agreement with everyone but will offer this thought: the functional things like cups and teapots are probably too much work to save but the platter forms may have a new life. As they're mostly decorative anyway maybe, at a later date, you can allow yourself to throw caution to the wind and decorate them in a way you never imagined. Maybe buy some low fire glazes and see what they look like, maybe throw some latex paint on a few. I followed the link to your website, you do beautiful work. Good luck at the show!

I see your point Hulk. I was responding to the "make it thinner" and "add more deflocculant" comments. (In addition to the "slumps in" aspect.) If the slip is properly balanced, generally speaking, it will flow out of the mold evenly. Maybe a slight ridge where the drain flow occurs but nothing that affects the shape to any degree. With too much deflocculant the slip stays wet longer and drains out poorly/unevenly. (And then because that spot stays wet longer it tends to deform more easily.) In theory you could rotate the mold but the slight complication there is that you will spend a lot of time rotating because the slip may take 5 or 10 minutes to set up. (Until it sets up it will still move in ways you want, or, in ways you don't want.)

Good Morning Seesee, Welcome to the Forum. At this point I would say you may have added too much deflocculant. Try adding water instead. The condition you describe is sometimes referred to as "flabby". When there is too much deflocculant in the slip it will stay wet longer than it should. This will lead to deformation when you open the mold. With the right balance the clay will pull away from the mold and continue to stiffen until you open the mold. When there is too much water in the slip it will stick to the mold and crack before it pulls away from the mold surface. (Similar to images of a dry desert.) I've come to appreciate slip that is a little too thin with water. The form will probably be a little thinner than desired but at least its the form you want. When the slip is flabby the form often deforms as you remove it from the mold and even as it sits out drying on the shelf. Good luck!

And yet another suggestion: metal window screen. Lay a piece of metal window screen, presumably aluminum?, on a flat surface and gently move the unfired pot back and forth. When you're done simply lift the screen and gently sponge away the dust. If the screen becomes clogged simply pull it in opposite directions and the particles fall out. This works especially well when wanting to get a flat surface on a plaster form but I also use it with pots occasionally.

As everyone has suggested, with luck, the kiln sections are plugged into each other and the sections can be easily separated. If you're curious the folks at Skutt can tell you how old your kiln is based upon the serial number. My kiln is the 1027 predecessor, the 231, and they were able to tell me they purged the records for that kiln in 1975 so my kiln is at least 48 years old. (Still firing great with new elements.) Before you move the sections, it would be helpful, to tighten the bands holding them together. (Usually, a series of hex nuts on the back of the kiln.) That way the bricks will move less. It's also helpful to move the sections horizontally in case the bands are loose. As you don't know how tight/loose the bricks are it is also helpful to have a flat board nearby so you're not moving the sections very far without support.

The kind of a mold you're pressing into and the clay you're using...are two factors that can affect your results. Are you pressing into a deep three-dimensional mold or are you pressing into a shallow relief type of mold? Strangely, what I have found, is that pressing clay into a mold is not as simple as I previously thought. If the clay is too wet, it sticks, if the clay is too dry, it doesn't fill all the deep details as well. A little trial and error helps. Perhaps your clay is too wet. A little stiffer clay is harder to press but less likely to stick. The way you press the clay into the mold also could be a factor: are you simply using your fingers to press the clay? I've seen where people use mallets to really FORCE the clay into the mold. (This for shallow reliefs with lots of detail.) Also, are you dropping a ball of clay into the mold and then pressing it from there? An alternative could be to roll out a slab of clay and then drape the clay slab into the mold. Doing it this way shortens the time the clay is in contact with the mold surface. The kind of clay being used also affects the process: a wheel throwing clay tends to be more plastic and friendly to manipulation, whereas a hand building clay might be more prone to cracking and less friendly to repeated pressing and pulling.

You might refer to it as a "thermally insulated oven" or "chamber"? Presuming you're in New York, you might google "NY Building Code kilns" and see what you get. (Just change the state if you're not in NY.) My state, Minnesota, publishes its building code on a public domain website. Its the full code, so it takes patience to look through the whole thing, but if you're lucky it's indexed well so you can find it sooner than later.

I'll amend Min's comment ever so slightly: Pre-covid, my local plaster suppliers, were able to deliver plasters that were fairly fresh and within the 6 month time frame Min references, Post covid, that has changed. Sometimes I've purchased plaster that was 4-6 months old. No ideal but I'll suggest a new timeframe of 6-10 months. (from the date on the bag)) Old plaster, having gone through a humid period (seasonal changes in Minneapolis for instance) will set quicker than fresh plaster and MAY be softer. Plaster that is several years old is best used for making solid plaster objects which won't interact with clay. A possible solution would be to drape the plaster surface with a thin cotton sheet. The plaster will still absorb the water but the clay doesn't interact with the plaster surface directly.

Presumably it will work but the question is how long it will take to set up and how quickly do you want the pieces out of the mold? Without fail I cast for 30 minutes, and 60 minutes after that, I have a soft clay form out of the mold. (When using plaster.) You're looking for faster turn around? The one thing you could try, to speed the process, is heating the bisque bowl before using. (or during, or after?) Good luck!

A couple things: plaster - if mixing 70-100 results in a thick mixture, and you only mix for 2-3 minutes, its likely you have old plaster. Adding more water helps with the pour but it will result in soft plaster. (Also takes longer to set up.) New plaster would be a good start. slip - If you don't know if, or what, deflocculant was added to the clay before you purchased it, it may be best to first attempt to make a slip with a decent flow. Add enough water to get a decent flow and let the mix sit overnight. See how much it thickens overnight. In the morning add some more water if it needs it. Its been my experience that if the slip is over deflocculated it will gel up in the mold no matter how thin you mix it. If the slip has too much water you will mostly get an extremely thin casting. I've never gotten a lumpy pour with too much water. Lumpiness is mostly a case of deflocculation issues. Mold soap - did you allow the soap to dry before pouring the plaster? If it was still wet it could seal the surface of the plaster and slow the absorption of water. Laguna Clay sells their B-mix in a powder form in a "Casting" version. They list the recipe on their website. What their website does not indicate is that the "Casting" version also contains the dry form deflocculants, soda ash and barium, already mixed into the dry powder.

I missed the question about plaster the first time I read the question. Like Min suggests a "pottery plaster" is your best bet. #1 Pottery is United States Gypsum (USG) pottery plaster. Georgia Pacific (GP) also makes a pottery plaster, K60. They're the same thing so either works for your application. (For many years #1 was only available in 100lbs. bags so I switched, to K60, when my back told me to do so.) Seeing that you are posting from CA I went to the Plainsmen website and looked up M340. It looks like they made some changes to the clay body, recently, so that MAY explain some of your problems. Plasticity can sometimes be a fickle thing so testing different making methods is really your only solution. The website comment, "You should have few problems drying smaller pieces but care and attention are necessary when making larger pieces" kind of jumps out at me. If it seems like an endless process perhaps another clay body might be worth trying? Good luck

Oddly, this looks like the kind of flakes you sometime see on old plaster molds used in slip casting. I wonder if a deflocculant type of substance, barium, soda ash, or sodium silicate, got mixed into this clay body by mistake?

AIM 88T cone sitter kiln for sale in Minneapolis. Never used. $500

Welcome to the Forum Memurphyiii. Looks like an interesting method. Also looks tedious. An alternative would be to pour a thin slurry of plaster into a plastic sheet which is suspended in a catenary form. You'll want to experiment with different kinds of plastic, though. Plastic garbage bags tend to be super soft and simply conform to the plaster. Stiffer plastic does a nice job of retaining its "crinkled" look while holding the plaster. (Think Saranwrap or stiffer.) I've tried it a few times, actually, when I have excess plaster from a pour. It creates a very irregular shape. Then make a plaster mold of the resulting plaster form. Nice thing about this method is that you can compare forms. Sometimes the resulting form has lots of wrinkles and is very complex/cool and sometimes too complex and too hard to mold. (i.e. too many undercuts) You need to make sure you're pouring just a thin plaster slurry when you make that initial form. Otherwise, the plaster just pools at the bottom of the plastic and you have a simple rounded plaster form. Good Luck

Welcome to the Forum Ronfire. Do you have a picture of the plastic form? Do I understand correctly that you slump clay OVER the plastic form and its cracks? While Pam is one solution you could, as well, try covering the plastic form with cloth, so that the clay shrinks against cloth surface rather than the plastic surface itself. (Take the Pam aspect out of the equation.) Plaster - if the form meets the tabletop at a sharp angle you may still have cracking issues. (Even if you make it in plaster.) Are you making platter forms with feet? This adds complexity to the slump mold process.

I've spent years trying to find a crystal clear glaze and have yet to find one. (Cone 6 porcelain) It could be that I'm applying it to porcelain but I'm not sure? Just when I get a beautiful clear glaze I see crazing. Just when I reduce the crazing I get cloudiness. It seems never ending. I find adding 5% Silica and Wollastonite to Continental Clay Cone 6 clear does a great job of solving the crazing but then the cloudiness kicks in. (Ever so slightly.) It also seems to depend upon what you're covering? Are these glazes going over underglazes/slips? Certain underglazes play well with glazes and some don't. I have yet to find clear glaze that plays well with Amaco Velour Black. A beautiful jet black color but which brings out the cloudiness in a glaze. Years ago, when I first started casting, in the 80's, I cast using Duncan Earthenware slip. I also used their Earthenware Dipping clear glaze. Most amazing clear glaze I have ever seen. Water clear and a beautiful glossy surface. I'm sure its very expensive but if you want "true clear" I'd look into it.

Here's the curious thing learned at the pottery shop: Mason Color (the folks behind Mason Stains) strongly suggested EPK to solve a "glazes too thin" issue. The gelling was seen as a benefit as it allowed the glaze to be applied thinly and yet get good coverage. (For the most part.) This change was motivated by an unusual experience with #6: a few months after covid lockdown we got a batch of #6 (maybe about 300lbs.) and it was terrible. It was almost yellow in color and it gelled up incredibly. And worse, it continued to get thicker, as time went on. Unfortunately, the young glaze crew, just added water to thin the thick glaze. This led to glaze cracking before the firing. It was really a mess for a while. EPK somewhat solved the problem but another batch of #6, a few months later, was much better and we returned to using it as the kaolin component.

The recent post makes me wonder if anyone can explain the differences between 6 and EPK? At the pottery shop we found that it was best to let a glaze mixed with EPK to sit overnight to let the mix thoroughly wet. Otherwise, upon mixing, the EPK glaze tended to gel up. (Get thick, in other words.) Whereas glazes mixed with #6 tended to mix up beautifully from the get go. The method for measuring consistency was to fill a 500 ml beaker with glaze and weigh it. The 6 glazes weighed fine but the EPK glazes weighed off the charts until they sat a day. I read that EPK is actually "washed" before it is processed whereas 6 is just dug out of the ground and bagged. (essentially) Does anyone know if this is why the usage difference?