Jeff Longtin

Hulk gives good advice, however, there is nothing that says you have to fire on a kiln shelf. You run the risk of the glaze running onto the kiln floor, but if that's a problem you can live with, firing on the floor is no problem. Does your glaze run? A kiln shelf helps to distribute weight if your piece is heavy. (If your piece is heavy it might stress the bricks if fired without a shelf.) You might also check to see how much your kiln lid sags. (If at all?) This too affects how tall you can fire. Its been my experience that most clays shrink about 5% from wet to bone dry. (At least my cone 6 slip cast porcelain does.) Doing a little test will help you determine the amount of shrinkage of your clay. As Hulk suggests leaving 1/4" space between the top of the piece and the kiln lid is good. I might make it 1/2" if I'm unsure?

The Bright Angle, in Ashville, NC might be someone to call? You might also try KleinReid in New York.

This brings back memories. (Thanks Peter!) I was a budding moldmaker, in 1991, when I marketed my services to ceramic/paint your own shops all over Minneapolis/St Paul. Coming from a Leach/Hamada orientation it was a shock to the system to see people making such things out of clay. But make they were. One ceramic shop, Judy's Ceramics, had a warehouse full of such molds. A WAREHOUSE! I had never seen so many molds. Judy had a thriving business making and selling pieces like this. (She was also a very nice person and supportive of my efforts.)

Don't know if anyone remembers when CM featured recipes for "flameware" bodies but here's one from that period. From Bruce Kremer, a grad student at Alfred Porous Flameware Body Petalite 25 Spodumene 25 Kaolin 25 Tennessee Ball 25 Darvan .7 If anyone tries mixing this recipe please post your experience. No idea what "petalite" is, or how it acts in a slip body, but I'm certainly curious?

Hey Erika, When you say "trying to pry them art" what do you mean exactly? How did you pry them apart? Another technique is to use heat/cold. Rather than use an actual tool to "pry" them apart use warm water. The warmth will cause one sheet to expand, slightly, and they may separate easily. This is how you separate teapot covers that stick "slightly". Fill the teapot with cool water than hold the cover end under warm water. If you're lucky you'll hear a slight "ping" and the cover will separate easily. The same method may work with your thin sheets. (Apply cool water to one side and warm water to the other. "Ping" and you have separation.) If you go extreme and use cold water and hot water, in an attempt to speed the process, you'll probably hear "CRACK" and know you went too far.

Just a few thoughts...I remembered the proper description of the diesel engine thing the company produced...they made diesel engine particulate filters. Simply put the filters are these ceramic things, 3" wide by 14" long, that have a 1/4 radius cylinder shape, and have 99 little holes that trap diesel fuel particulate as the exhaust fumes pass through. The extruder that creates the holes is made up of 99 little tubes (I think?) that push through the clay and leave it with 99 little holes all the way through the 14" length. The holes were 1/16" if I recall. Truely amazing feat. The extruder form was metal. And I think it was heated? (for lubrication perhaps?) Maybe this is the key to this situation. Rather than think of terms of plaster/cement molds think about heated metal molds? Extruded rather than cast?

Leon Nigrosh wrote a nice book about "low fire" back in the 70's that I always found helpful. I think I mixed his recipe but don't remember the outcome. It is: talc 45, ball 19, feld 19, kaolin 17. with soda ash .15 and sodium silicate .15. You could also try it?

An additional thought on Rhodes: he wrote several books, Clay and Glazes may be the most well known but he also wrote "Stoneware and Porcelain" and "Pottery Forms". Our high school library had all three books and I poured over them endlessly as an impassioned wheel thrower. What made Pottery Forms so unique was that Rhodes often showed pottery shapes in profile. Meaning...he would cut the pot in half to show the wall thickness. That really sent home the message that a well thrown pot has an even thickness clay wall. (To prevent cracking, mostly, and warping.) I don't think too many other books had this element? What the profile shots also did was send home the message that crafting a good pot involves two processes: throwing the pot and trimming the pot.

Hello Hotzn, Welcome to the Forum. Your questions are a bit too technical for easy answers. You would really need to decide which clay you want to use and then run your own tests to determine "draft angle". ("Draft angle" = the taper necessary to allow for one piece to be pulled from another.) Different clay bodies have different levels of shrinkage. I'm not sure which would be more advantageous for you, clay bodies with lots of shrinkage or clay bodies with less. Those factors then affect how easy the body is to keep in suspension. (On a consistent basis.) Years ago 3M had an Industrial Ceramics division and they commissioned me to make a simple mold for a simple piece.(For engine cylinder liners.) We never got into clay formula discussions. Years later, when I reached out to my contact to ask a technical question, I learned that he had retired and that they sold the division to a company in Mexico. More recently another local company, a branch of a Dannish company, approached me about making molds for silicone carbide kiln furniture. That project never got off the ground but they did show me the process for making another ceramic product they made. (Deisel engine air filters.) The "clay" was black and made with silicone carbide powder. They extruded the forms in a very precise extruder device. They fired it, however, in a nitrogen fueled "kiln" and fired them to 3000 degrees. Thats what I envision you're looking for but that's super technical stuff and I have no idea how where you'd begin on that. Good luck.

Both USG (Untied States Gypsum) and GP (Georgia Pacific) sell their products in 50lbs paper bags. If your plaster was in a plastic bag, in a cardboard box, it means your supplier is repackaging the plaster. This prevents you from knowing hold old your plaster is. (All 50lbs bags are dated with the factory "packing" date.) If it's not too much to bear I would suggest you buy a 50lbs bag. That way you will know how old your plaster is and you can learn about the effect of age on plaster. The fact that your plaster mixed quickly means it is old or it has been exposed to moisture at some point. If you buy your plaster in 50lbs bags you can better control how the plaster is stored.

My guess is that its a foundry type kiln. A crucible is set into it and the burners generate heat to melt some material: steel/glass? The five burners would heat the material quickly. (and/or uniformly?) The square hole in the top allows for some sort of metal apparatus that enables two people two lift the lid off while keeping a distance from the intense heat. Perhaps a glass annealing kiln?

Compression - it's not that your compressing/squeezing the clay as much as you're ALIGNING the clay particles. (That's the benefit of a rubber kidney tool.) In the unused state clay particles are randomly aligned. (Or so I'm told.) Wedging helps to align the particles a bit. Throwing aligns the particles some more, and using a rib tool aligns the particles even further. The act of aligning helps to reduce the chances of cracking. Circular cracks indicate that the clay is shrinking in circular directions. If you rib a form, before you pull it from the wheel, you align the particles in a horizontal direction and counter the circular direction the particles want to follow. (If that makes sense?) Too much water, in the throwing process, contributes to more shrinking. Less water = less shrinking. (This explanation may have a few hits and misses. I know what works from a usage point of view...the technical explanation has been learned over the years.) The other aspect, about bowls, that contribute to cracking, is irregular clay wall thickness. As much as possible its best to trim a bowl such that the outside profile matches the inside profile. (That way the clay shrinks evenly.)

Hello Garden Rock. Welcome to the Forum. What are you trying to do? Clay pieces, cement pieces? Ultracal 30 is a USG product. The bags have a "packaged date" on them. What is it on yours? It sounds like you may have old cement. Anything over a year old is poor quality. 6-12 months will be problematic. Under 6 months your mixing method may need to change.

Hey Suresh, Been down that road but it's been years. No recipes but I do recall most earthenware casting bodies have lots of talc in them. Did you see that Laguna makes a low fire casting body? MCP Artware. It is described as a "high talc" casting body. I'd try that first. Good luck

Hulk/Tom, When people throw pots they generally lubricate the clay with water. (So the clay slides between their fingers.) The water will often collect at the bottom of the form. If the thrower repeatedly tries to "improve" the form they may leave a lot of water at the base of the piece. If they don't keep this accumulated water to a minimum the clay may become over saturated and separate once the piece dries/is fired. An easy way to rule this out is to throw the form quickly and keep the water to a minimum. If this type of cracking persists it may be a clay formula problem. Rather than try to throw the perfect shape, and over work the clay, throw a pot as best you can, in a shorter amount of time, and then trim it to your desired shape.

If you do a search on the term, "death wish coffee mug" you'll see a hand thrown mug with a simple medallion. One thing you'll notice is that it has a border. The border is nice because it defines the image. You'll also notice that the image is all raised. The letters are raised and the artwork is raised. The background, however, is recessed. Thats nice because it simplifies the finishing process. After glazing, the high points of the medallion are cleaned, with a sponge, such that the white clay contrasts with the dark glaze. It means that some areas of the medallion are unglazed but it gives the best contrast. Creating a medallion that communicates a specific image is not easy. It takes practice to understand how the process, assembly/glazing, affects the end result. A little experimentation might give you a better idea how your process will affect your results.

Interesting video. Thanks for posting, Peter. I disagree with a few of his methods. For some reason he sprays a release substance into the silicone mold before pouring plaster. If I'm not mistaken there's no need to do that. My urethane rubber molds have lasted 10 years and they release easily without any release substance. My other disagreement is with the wooden frame concept. (For the rubber molds.) Rubber comes in a variety of hardnesses. Polytek recommended using a urethane rubber with a hardness of 55 and it works very well. (7455) It keeps its shape and it flexes when it's time to remove the plaster mold. I've used the 10 yr old molds to make 60 molds to date. I'm not patient enough to gently remove a wooden frame 60 times and have it retain its original shape and precision. (If this gent does than he's miles ahead of me in that regard.) I do appreciate that he has had the fortitude to make videos to show the process to those getting started.

Hey Betty, I didn't. I think I tried her cone 7 recipe but went with something else instead. I had Lithium Carbonate, in my studio, for many years. I do believe I used a recipe, with it, in it. Once Lithium was viewed as toxic I decided to abandon that recipe and turned to premix glazes instead.

Another recipe that I have used is one published by Ceramics Monthly in their 1982 edition. In that edition they profiled Dorothy Hafner, a potter in New York. I tested out the recipe and used it for a year, or two, until I realized my clear glaze was crazing on it. OM4 10 EPK 20 #6 20 Nep Sye 25 Flint 25 The original recipe calls for 350 cc N brand Sodium Silicate and 50 grams Soda ash (to 500 lbs powder?) but I found the Darvan #7 proportion I listed above to work equally well. For kicks I mixed up a small batch last year and was surprised how nicely it casts and how white it is.

Over the years I have, on occasion, attempted to clean a bristle brush, that is covered in porcelain slip, by brushing it against a plaster mold. The resulting thin layer of slip dries quickly and shrinks from the plaster surface quickly as well. I would suggest this method for producing a thin porcelain slab. Give it a try. You'll find it an interesting method to play around with. You have to brush on additional layers soon after the previous one has stiffened but with patience you can create thin porcelain sheets. How you prevent them from warping, as they dry/stiffen, is completely up to you but at least I've given you a starting point. (Are you looking to make flat sheets or is a slight warp acceptable?) Good luck

Good Morning Betty, I'm sorry I missed this. (Its been a busy last few weeks.) A website called "Claybucket.com" has some good recipes. My favorite is: Water 10000 gms Darvan #7 80 gms Bentonite 100 gms Grolleg Kaolin 11000 gms Nep Sye 4600 gms Silica 4400 gms When mixing Bentonite its usually best to mix it with the dry material first to break it down. If you just add it to water it will clump up horribly. Start by adding the Darvan to the water first. Then I add the dry ingredients. It's been awhile but I think I add the Nep Sye and Silica first and then the Grolleg Kaolin. As the slip will thicken quickly I find it best to add the clay a little at a time. I hope this helps.

When I was a young mold maker I read every mold making book I could find. Most were British and oriented toward "factory" methods. (And all referred to molds as "moulds".) Out of all the books I found only one mentioned leaving the mold soap wet while you poured. So I tried it. The results were terrible. (Like the results Giorgio had.) Afterwards I decided to wait until the mold soap was completely dry before pouring. If memory serves me right none off the books actually said, "wait until the soap dries." They all talked about using it but none actually discussed its usage in actual practice.

The experience you're having is common. It really depends upon your clay and which stains you have available. For consistent results you would be better to stick with cool colors. (Blue for instance. ) It will also require testing on your part. (Lots of testing.) Generally speaking, at cone 6, you will only get "soft" color. If you want an opaque saturated color you should try lowering your temperature. (Try using an 04 earthenware clay body instead.) For an idea of what's possible in the realm of colored clay you can look at Josef Wedgewood "Jasperware". He made it many many years ago before Mason was a company. It will give you an idea what colors are easiest in this area.

Good Morning Lilith. To answer your question: Yes, I've been there before. For nine years I worked at a pottery shop with 18 1231PK's. Ten were dedicated glaze and eight were dedicated bisque. As a majority of the crew were inexperienced young people it fell to me to figure out "why the kiln isn't working". We had one glaze kiln like yours. Changed out the elements, changed out the touchpad, maybe replaced a relay or two. Replaced the thermocouples. Seemingly everything. (At Perry's suggestion.) For some reason could not get that kiln to glaze temps regularly. We moved it over to the bisque kilns but even then it continued to fire inconsistently. (But as a bisque kiln the inconsistencies weren't as dire.) It sounds like you know what you're doing but I'll mention the simple things that were often a cause for problems at the pottery shop: when you have a 1227 sitting next to a 1231 its easy for employees to mix up the elements sequence. We'd shoot for 28-22-28 amp sequence. If the amps ever went over 28 I knew someone probably put a "Top/Bottom" element where "Intermediate" should go. The crew liked to pack the glaze kilns tightly. Sometimes I'd see a pot, or a kiln shelf, resting against the thermocouple. Because the kilns were packed tightly sometimes bits a glaze would pop off and land on an element. At high temp these bits would burn through the element and cause a failure. Sometimes folks would replace elements without removing the glaze bit. (Which usually melts into the element groove.) I would have people carve out the melted glaze so that nothing would interfere with the element when it got hot. These are just a few thoughts to get you thinking in a different direction. Sometimes a kiln problem is really obvious and sometimes not. Good Luck!

Good Morning Kathryn, fyi: typically the cones are set in the opposite of what you have done here. That way they can melt and not influence the neighboring cone. (So when 7 melts, there's no chance it will fall on 8, and affect it.) The sequence results in a shape that looks like the Sydney Opera House. (The first cone bent alot, the middle cone bent a bit, and the third cone just starting to bend.) Digitalfire has a great page about these things. Pyrometric Cone (digitalfire.com) While Orton suggests a specific angle at which the cones indicate specific temperatures I've found cone plaques, and what they tell you, to be more subjective. If your glazes look beautiful when fired to these temps than thats the bend you want to maintain. If your glazes seem overfired than you would want to lower the temperature a bit. The other thing that cone packs help you understand is consistency. These two cone packs look fairly similar. Thats good, If they're two different firings or two different areas of the same firing it looks like you have a measure of consistency. So thats good. I always look at the "guide" cone to decide if a kiln "overfired". If the guide cone has bent alot then its a definite overfire. If the guide cone has bent a little then its not a bad thing. It really depends upon what the glazes look like. Are you happy with the glazes?