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Jeff Longtin

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    Mold making, slip casting, porcelain,

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  1. John Ballesteri(?) a professor in Idaho, developed a clay formula for ceramic 3D printers. (The printer lays down the slip and then the piece is fired as normal.) A professor at Harvard(?) developed a printer that prints molten glass and fires it as the printer head moves along.
  2. As I often give mold making advice I'll share the form that brought me to mold making in the first place. (From a passion for wheel throwing.) The original teapot was much larger. This smaller version took a few weeks to make.
  3. Nata, Pinholes in casting slip are the result of too thick a slip. Its a delicate process but you can thin a slip two ways: adding more water or adding more sodium silicate. If you add too much water you may see more cracking in the mold. If you add too much sodium silicate you may see the slip gel up too much. Thinning the slip with water is usually the best "first step". Another trick is to moisten the mold surface with water before casting. This was a trick given to me by a doll maker many years ago. good luck Jeff Longtin
  4. I was a student of Don's, in '82, at UW-Madison. (Normally they didn't let freshmen into his classses but I was persistent.) Don was a great guy. He had a lot of enthusiasm for making pots. Some of the grad students would help him fire his big pots, out at his farm, on the weekends. The students always came away with great stories and lots of excitement. There was a lot positive energy, in the crafts scene, back then. (An "anything is possible" mentality.) Chilhuly was pushing the boundaries at Pilchuck and Don was pushing the boundaries in Madison. He will be missed.
  5. The literature I've read suggests that drying plaster over 150 degrees causes the plaster to decalcify. As such I dry plaster in the 120-130 degree range. (Most forced air heaters come in at this range.) Air movment is also important. Like air movement causes our skin to dry out it also causes a plaster mold to dry out, as well. When I'm casting lots of molds, and need them dry sooner than later, I create a box form, with metal shelves or sheets of plexi, and put a little room heater at one end. (Creating a heated tunnel, as it were.) This gives me heat and it gives me air movment. I can have molds dry, in just a day to two, this way.
  6. In most cases the handles are cast seperately to eliminate shrinkage issues in the drying phase. This is probably not so much an issue for studio potters as much as porcelain factories, where they're pouring cup forms that are very thin, with very tight clay bodies. (bone china, for instance) I cast my braille cup handles seperately so that the handle is not hollow, which wolud fill with hot liquid when in use, and because the shrinkage would disrupt how the cup shrinks within the mold. On the otherhand, most of the cup molds that I've seen commercially had handles built into the mold.
  7. Most of the doll artists, with whom I worked, made their dolls with porcelain clay. They often cast thin. Typically the pieces weren't assembled before firing. (Usually they were assembled with cords, after firing, so the limbs were flexible.) Because the thin porcelain pieces were fragile most doll artists would fire them to 018 before cleaning the seams. As well, they would lightly sand the cheeks, with very fine sandpaper, to get a high sheen. (As the finished dolls were left unglazed.)
  8. To address a few of the questions raised on this thread: I find it best to make molds from pieces that are inert, such as a bisque clay piece or plaster. A mold can be made from a wet/leatherhard clay piece but the moisture from the setting plaster will often deform it. This becomes especially challenging when pouring a multiple part mold. Yes, a bisque piece has shrunk but only 1-2% if you fire at 06 or cooler. When casting products for potters I prefer they bring me a bisque piece. I then seal that piece with clear urethane. After a day, or two, I go ahead and cast a mold. The nice thing about casting from bisque is that it gives you the freedom to make mistakes. If your parting line is not as tight as you like you can repour the offending part. take care Jeff Longtin Minneapolis
  9. Thomas, Are you in a city that has a clay supplier? I live in Minneapolis, MN. At one point we had three shops that supplied casting slip. Today we're down to two. Both sell it in 5 gallon buckets. The slips that are available for doll makers come in 1 gallon containers. It would not be economical for you to make 300 pots a week using that form of packaging. If you are near a supplier that can deliver slip in 5 gallon buckets that is more efficient. As Mark suggested you can buy alot of used slip casting equipment on Craigslist and other sources. If you don't have a local supplier then mixing your own is the best bet. Good Luck Jeff Longtin Minneapolis
  10. Tim, Your molds are too thin. I just poured some rubber molds using Polytek 74-55 and they allowed me to cast 50 molds so far. The walls of the molds are 2 inches thick. $600 worth of rubber but they'll last 20 years. As far as terminolgy is concerned the original is considered the "model". The first mold, of the model, is considered the "master mold". The mold that you use to produce more molds is considered the "block and case mold". (The "block" being the mold face while the "case" is the box that surrounds it.) In the past the block mold was made of rubber while the case was made of wood or cement. Nowadays the whole mold, block and case, is made of urethane rubber or silicone. Good Luck Jeff Longtin Minneapolis
  11. Brian, Are you weighing your materials beforehand? Weak plaster is usually caused by high water to plaster ratio. I used USG #1 Pottery for many years and did not experience much chipping. I don't weigh my materials everytime I pour but I do when I get a new batch of plaster. (So I can gauge how old the plaster is.) Pottery Plaster and GP K-60 are the best for slip casting. GP Densite and USG Hydrocal are usually best for ram pressing. They can be used for casting but your set up time will be much longer. Good Luck Jeff Longtin Minneapolis
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