Jeff Longtin

My hunch is that there is too much sand in the clay. I would have suggested you test the clay first before adding that much sand. Has the sand been washed? I've never worked with sea sand but I wonder if it adds salt to the clay? Do you know the mesh size of the sand? A smaller mesh would have less impact on plasticity.

A couple thoughts: would it be possible to inflate the toy using water? Potentially you could freeze it , after its filled, to make it solid. You mention foam insulation...have you tried the foam tire inflation kits that bicyclists use? I'm told they work really well to keep a tire inflated. Another alternative could be latex rubber. It would flex with the form but you could apply several layers to develop strength. An extreme thought would be to put the inflated form into a box and then fill the box with alginate. Once set you would gently cut the alginate and remove the toy and then fill the alginate mold with plaster.

To answer both questions: M.A. Handpainted - Unless the bisque has been in a nice box and well wrapped I would suggest you wash it, and let it dry, before you paint it/glaze it. A fine dust can accumulate and spoil any surface decoration you apply. LeeU - Years ago I made molds for all kinds of folks wanting to make all kinds of things. A few of these folks were the "paint on bisque" crowd. Coming from a potters background it puzzled me at first but then I realized it made sense. If the piece isn't going to serve a functional purpose, and will be strictly decorative, why go through the bother of high fire. If bisque temp produces an item that will be permanent, without the higher temp firing costs, it makes sense to stay within the bisque range. Sure, a higher temp would make it more durable, but this was not a crowd that was looking for generational durability. They just wanted a decorative piece to put in their house. On the flip side, I did make a mold of a classic Belleek porcelain vase. It was the most delicate thing I've ever held and it had a crack. I reproduced it in modern porcelain and the customer was very pleased. Upon delivery she told me the vase had been in the family over 100 years. I met all types making molds and it was an eye opening experience.

I've been casting cone 6 porcelain, for years, and never had a problem with underglaze and seams/handles. As I cast thin I prefer to apply underglaze, Amaco, when the pot is bone dry.

Just to give you a heads up Lucy, if I'm perceiving the image correctly, it would appear you have a 1227. The "12" referring to the number of bricks running around the kiln. Just do a quick count before you buy elements. An element for a 1227 will be different than an element for a 1027. I can't recall if a 1227 has fuses but if it does those would be a simple fix. (50 amp fuses are about the thickness of your index finger and 2" long.) When an entire section is black its either a relay or a fuse.

To pick up on what the Pres mentioned...my gut tells me the slab is being overworked. (In the making process.) Too many "walls" are being attached and its causing stress in the firing. The walls may be necessary for strength and to hide things, but the attachment process is too stressful. Can I presume the slab is flat on a surface and you're pressing the walls into it? The question would be how can you reduce the stress when going thru this process? My gut tells me you should create a form that is intact on its own, essentially, and then put the shaped slab OVER it. That way taking stress off the slab itself.

Hello Meka, when you say "intended shaped mold" what are you referring to? Do you want to use it as the mold box, and pour silicone into it, or are you intending to actually carve a form out of styrofoam? I've not used silicone but have used urethane rubber. As long as I coat non-porous objects with silicone spray I've had no problem with sticking.

Minneapolis has a few pottery shops that could handle this level of production. If you want to shoot me some info privately we can discuss the possibilities. Big question would be your expected per piece cost and level of downpayment you would be able to make.

Good Morning Christy Ann. I have years of experience with MN Clay. When you mention Minnesota Clay 03 are you referring to Minnesota Clay #3 White Stoneware? It's been a very popular white stoneware for many years. Are you using the clay right out of the bag or is it reclaimed clay? Sometimes reclaimed clay can have bits and pieces that bloat in the glaze firing. I use Amaco underglazes and regularely fire them to cone 6. I use a cone 6 clear glaze over them. Is your glaze a cone 6 glaze? Do the bubbles appear to be clear, i.e. glaze like, or does appear that the actual clay piece has bloated/bubbled?

Mold Design decision - While you can make a multiple part mold for a complex form you need to also consider the usability of such a mold. Each part creates a seam.(That needs to be cleaned.) Each part needs to fit together, well, with its neighbor, so the mold holds together tightly. If you have a mold with lots of small parts you then have to be really careful when you work with the mold. If you crack, chip, or break just one part does it ruin the whole mold? For the aforementioned reasons it is best, if possible, to keep your parts to a minimum. (Less time spent cleaning, less worry about breakage.) Undercuts - A way you can keep detail but reduce its impact is to design the undercuts so they face one direction, either front or back. If you alter the undercuts so they follow the "pull" of the mold, as you separate the two pieces, then you can keep detail but make it work for you and not against you. Hope this helps.

Hello Malain, Welcome to the forum. How tall is this piece? You intend to have the final product be ceramic, correct? Stoneware or porcelain? In the picture there are several dark/shaded areas. Those would suggest undercuts. If you are willing to fill in those undercuts you may be able to make these with just a two part mold. (That would be best.)

Continental Clay makes an "Alumina based" kiln wash that is terrific. (They also make a "silica-based" kiln wash which I don't use.) As the recipe OldLady mentions is fairly simple, and universal, I presume the CC product duplicates it.

Hey Min, I looked at this but couldn't come up with an explanation either. Without seeing it in full context I'd only be guessing.

A little trick to separate teapot lids from teapot bodies...run hot water over one portion of the two pieces. If the universe likes you there will soon be a slight cracking sound and your lid will easily separate from the teapot. If the universe doesn't like you there will be a loud cracking sound and things will separate in a most ugly fashion. If a single application of hot water doesn't work than an application of cold water, to the second piece, followed by hot water to the first, should ease the process. It doesn't work, all the time, but most times it will.

Another option, though hugely tedious, is to keep the mold moving, after you have filled it with slip. That way the bubbles are kept moving and can't settle in the plate wall. You don't need to do it for the full 30 minute set period, but a few minutes, to move all the air bubbles, will usually suffice. Filling the mold only half way, during this period, is the most effective. Once you've swirled the mold a few times then fill it to the top. Years ago I delivered a few molds to a customer who used a "roto-caster" to cast large cement table bases. By keeping the mold in rotation they were able to cast large cement pieces with thin walls. I realized it might also work with casting slip and air bubbles. It does.

Great story Mark. Coincidentally I brought in a copy of "Clay and Glazes for the Potter", by Daniel Rhodes, this morning, to introduce a younger employee to the types of books I referenced in my younger days. I asked her if she was familiar with "Alfred" as being the center of the universe, in the pottery world, for many years. She nodded that she heard of it but wasn't familiar with it beyond that. It certainly had an impact, on this young potter, from afar.

If brush-on is a possibility I'd say the Amaco underglazes fill the bill. The LUG white has a nice cream color while the Velvet has a brighter white quality. Three coats and you can get a nice opaque surface.

Hey George, You've set yourself up for a challenging process. The only other thought I have is to cast the top portion/slab separately from the rest of the piece and then assemble them in the leather hard state. Perhaps by casting separately the particles would align in a way that minimizes warping in the horizontal direction? You could also add slivers of clay that could stabilize the slab? Good luck!

Is it safe to assume the piece is about 30 cm wide? That's a large diameter for a slip cast slab of clay. I would suggest a smaller form within the piece supporting the top. Ideally the bottom surface, of the top slab, would be unglazed. I do this on several pieces. I apply kiln wash to the top of the inner support and the clay piece comes off easily. A bit of a pain but it almost entirely eliminates slumping of wide, and flat, slip cast forms. ps. after the inner support form has been high fired, cone 6, I keep it on the shelf and re-use it when necessary. The kiln wash prevents it from sticking to the piece of the kiln shelf.

Welcome to the Forum NotaBot. There are several reasons why a glaze might "pop". Not so much the glaze itself but something "other" thats gotten into the glaze. Perhaps while you were glazing you set the unglazed pot on a dirty shelf such that something then got into your glaze when you dipped the pot? Perhaps there is a contaminate in your clay that exploded when it was heated? Perhaps you applied several coats of glaze and they had an unexpected reaction to each other? I've been making cone 6 porcelain pots for 30 years and rarely had this happen. (Mostly single dip.) These last few years I've been working in a pottery shop where we use cone 6 stoneware glazed with multiple layers of glaze. We loose elements regularly due to glazes popping.

At first blush this looks like the rippling that occurs between the top and bottom halves of a two part plate mold when the slip is too thick. The air bubbles have no where to go so they stay trapped between the two surfaces. At some point a vacuum is created and it sucks the clay wall inward. I never knew it could happen until I started casting two part plate molds myself. The solution is to either thin the slip or redesign the mold? I chose to redesign my plate molds instead.

Years ago I was watching an episode of "This Old House" when they highlighted the process of manufacturing terra cotta planters. I was amazed when I saw that they were pressed molded using a metal mold. What allowed the clay to release from the metal mold was that the mold heated up and the water turned to steam. The steam allowed the pot to release easily from the mold. I'm presuming heat is the missing ingredient in your situation. (Or you need to press when the clay is stiff and less prone to sticking?)

Hello Daisy. Welcome to the forum. If you fired your pots to bisque temps, before you glazed them, you shouldn't have a problem. I fire my cone 6 slip cast porcelain on High, from start to finish, with no problem. (with a cone 06 bisque beforehand.) The elements lasted 25 years so I saw no ill effect. Definitely do not want the pots to cool to room temperature. Re-firing a partially melted glaze can sometimes create terrible results.

Hello Kel-Kay, Do you have a gram scale? I would weigh out 100 grams of slip and set it aside to dry. After it has completely dried, I would weigh it again. Then determine how much powder makes up each 100 gram batch. Then you can conduct simple small tests. In my own situation I determined that the water is about 50 grams per 150 grams slip. (In other words: 50 grams water to 100 grams powder.) I use this simple ratio when I add mason stains to my premade porcelain slip. (I further determined that 150 grams slip equals 3 oz liquid. ) Recently I tried adding 3% Nep Sye to my clay to see if it would reduce glaze crazing. I measured out 9 oz slip and added 9 grams Nep Sye. It didn't achieve the desired effect so I tried firing the clay to cone 8. That worked much better so I chose to go that route instead.

For those with a similar question I'll share this: soda ash is a very strong deflocculant. It is rarely used on its own and commonly paired with sodium silicate. ("N brand" as I know it.) From what I've read the sodium silicate works on the easy to defloccuate particles while the soda ash works on the harder to deflocculate particles. (As there aren't as many of those, in a casting body, you don't need as much.) Darvan 7 and 811 really came on the scene, for potters anyway, in the 80's after the publication of the book, "Moldmaking for Ceramics", by Donald Frith. (A wonderful book for those just starting to make molds.) Until the book came out most casters used the sodium silicate/soda ash combo to deflocculate clay. (Also adding barium carbonate to iron containing casting bodies, to reduce scumming.) The other reason soda ash is only used in small proportions is that it is very caustic. Caustic to humans and caustic to plaster molds.