Jeff Longtin

I was recently asked about a pouring box for mold making. Here are two versions I use in the studio. The image with the bulbous object is my main pouring box. The pieces are 3/4" plywood cut into 5"x15" boards. I then screwed 1"x1" strips onto the ends which provide a lip for the small "C" clamps. Very quick and easy to assemble a box in any dimension. The second image shows another box variation that I use often for flat castings. The sides of the box are 2"x2" pieces of pine. (In his case I also used some 1"x2" strips.) The 2x2 pieces are labeled as "turning blanks" and come in a variety of wood types. (Woodworkers use them to make stairway balusters.) Because they are square and stocky they don't need much bracing other than a little clay at their base. The mold on the left I free formed and did not use the box to limit the plaster. (I shaped it by hand.) The mold on the right I filled the box with plaster and then ran a straight edge along the tops of the wood pieces which resulted in a mold that was evenly thick.

Years ago I made plaster molds for a customer who then used my molds to make clay tiles. The clay tiles would be fired and then used as COOKIE molds. (The tiles had a slight relief that would make for very interesting cookies.) The process, I was told, was to lightly oil the fired tile and then place a little bit of cookie dough on it. Put both in the oven and bake for the normal amount of time. As the dough softened, and baked it, would lay down on the tile and take the relief. Then, once the tile cooled enough, the cookie would lift off the tile easily. I've often thought of making promotional cookies, for studio events, but haven't taken the time to do it yet. Making chocolates sounds fun but challenging. Let us know how it turns out.

Looks like Lemon Grove is in southern CA, near the Mexico border. (If you google it you can actually see the storefront.) It appears to be located near a place called Plaster City. (A site producing plaster for USG.) Imagine setting up a mold making studio in Plaster City?

I have mixed up the "Magic Water" recipe and it works. One gallon water, 9.5 gm sod silicate, 3 gm soda ash. Soda ash is quite caustic so make sure to wear gloves, or a glove, if you have a cut on your hand. It does sting otherwise. If I recall the "clay" content was 2/3 clay powder and 1/3 toilet paper.

Hey Jeanne, I've had similar experiences. The Velvets are nice, because they don't cause the overglaze to dry out/pinhole, (as much) but they are thin. The LUG underglazes are nice, because they're more opaque, but they cause the overglaze to dry out sometimes. An alternative is to use the LUG White Underglaze, as a base, and then add Mason stains for color. This way you can control the intensity of the color and still have great brush-ability. While SOME Velvets/LUG underglazes do well at cone 6 I presume most are really intended for cone 06. By using LUG as a base, and adding your own Mason stains, you're creating an underglaze that can tolerate cone 6 and beyond. (possibly?)

If you look in the Community Marketplace someone posted looking for a used slab roller. (In Boulder no less.)

Years ago I found a book about Belleek porcelain. (I had never heard of the company before finding the book. Apparently, it's an old Irish pottery.) In the book they showed beautiful porcelain figurines. To allow for variety each appendage, arms and legs, were made with a ball end. The ball end fit into a socket on the main torso form. That way the arms and legs could be arranged in different ways. A huge pain to mold but I did this for the Akita mold. Each leg had a ball like end that fit into sockets on the main torso. As the piece was small there wasn't much room, to move each appendage, but it did allow the artist to play with it as he saw fit.

I find it best to thin the underglaze to a heavy cream consistency. (or slightly thinner) When I open most underglaze jars they have a pudding like consistency. Even though the mixture is incredibly smooth I find it goes on incredibly thick. (Hence brush marks.) As well, I apply coats 5-10 minutes apart. (That way the surface is still slightly damp.) If I wait until the previous coat is completely dry I find it hard to prevent brush marks.

Beautiful work Hyn Patty. Can't imagine molding it. Years ago a gent sculpted an Akita dog with as much detail. About the same size as your project. Nightmare project but he was happy with the molds when it was completed. (He was a breeder and brought several dogs when he picked up the molds. Super strong animals but very well behaved.) Have you ever heard of Breyerfest? For several years I worked at a pottery shop that made promo mugs for the event. Had no idea what it was until I looked it up. Here in Minneapolis the big collectible was little porcelain buildings made by Dept56. Had no idea there was another type of collectible out there.

Hey Paula, How big is your plaster board and how old is it? You can "repair" plaster but I find it only works best with new plaster. Presumably your plaster surface has absorbed a lot of minerals, from wedging lots of clay, and that makes it less friendly to new plaster. If you want to try it simply mix plaster, into a milk like thickness, and pour it into the plaster holes. As soon as it loses its wet sheen use a sharp scraper to level off the surface. Do this procedure a few times until you have a smooth surface. It may not absorb as well as the rest of the slab but at least the holes will be filled. Also, is it not possible to flip the plaster board and use the other side as a new surface?

Hello Karen, Can you post pictures? From my experience only plaster molds can be used with slip casting. I do know folks who have used rubber molds for pressing wet clay but that has its own challenges. I would recommend you make plaster molds, from your silicone molds, and then use the plaster molds to slip cast.

Hey Samantha, Welcome to the Forum. The kiln looks great. The outside jacket looks fairly clean and the interior bricks look to be in good shape. The kiln floor looks to be in good shape. $450 is probably a good price these days. It uses a kiln sitter to control the kiln so that's just one aspect that you will need to consider. (You have to manually turn up the kiln throughout the firing process.) I've been firing a kiln, very similar, and have had no problems for 25 years.

It's really important to remember that when mixing larger amounts of plaster that the longer you mix, with a mechanical device, the stronger your plaster will be. A slow mix will result in soft plaster. When I mix large amounts of plaster I run a small spade thru it to break up any lumps BEFORE I add it to the water. (That way it "wets" quickly.) I get the plaster in the water sooner than later than I spend my time mixing with a "jiffy mixer" type of device. Old plaster - precovid 4-6 months was considered "getting old" but post covid I'm lucky if I get plaster that is less than 6 months old.

Hello Klindsay. Welcome to the Forum. Yes, you can make a mold of a bone dry clay form. It shouldn't dissolve. (As you suspect.) It will/might soften, however. That' where it could get complicated. Years ago I made a mold of a seven piece doll form. (In total the finished form was about 24" tall.) I got to the project later than expected, as in weeks, and the pieces were mostly bone dry. Out of caution I applied Murphy's Oil Soap to all the pieces but I think that was unnecessary. While the pieces didn't dissolve they did break apart when I tried to remove them from the molds. The client was very unhappy and I was very unhappy. She didn't get the molds she wanted and she didn't pay me the full amount. Since the forms you want to mold are now bone dry I would suggest you bisque fire them before you mold them. After the firing you can seal them with a spray acrylic paint and they will survive the molding process and allow you to remake the molds if necessary. Good luck. Making molds is never as simple as most people think.

Hello River, It looks like you may have gotten more responses if your question were posted in a different category? That said, I'll offer this: It sounds like you intend to hand-build or throw more porcelain pots. Is that correct? You use the term "slip". I presume you are referring to slip used to assemble two clay objects and not casting slip? Casting slip has both sodium silicate and soda ash, usually, and those two defloccants aid in assembling slip cast parts. Those two ingredients also aid in making a slip used to assemble thrown or handbuilt pots. I first learned of something called "magic water" a few years ago and realized it is a very good bonding agent for clay pieces. (I've never had a problem assembling slip cast parts as I just use casting slip to do so.) A recipe, for Magic Water, that has proved successful includes 1 gallon of water to which 3 tbl of sodium silicate and 1-1/2 tsp soda ash has been added. (This in addition to clay powder.) I've never used Magis Water without clay powder. I've always used it as a diluted form of casting slip essentially. Does this answer your question? Good luck! Porcelain is a wonderful clay body to use.

Plasticene vs plaster forms - You can make plaster molds from plasticene forms but the heat of the setting plaster may deform the plasticene original. That really sucks when the client spent months creating the form. After you pour the first half of the plaster mold you will need to apply Murphy's Oil Soap to the surface to seal it. Brushing around a soft plasticene form takes a delicate hand. The first few times I did it I left brush marks on the plasticene form. Working from a plaster form removes those challenges.

Hello Ugly Mug, welcome to the Forum. The video Peter posted does a good job of showing the process. Creating a bed is the best "first step" in the process. Like the video I choose to make a bed using foamcore. (Its cheap and flat and clean.) I use clay to seal the gap between the shape and the foamcore. Every so often I coat my plywood pouring boxes with Johnson Paste wax. It won't prevent the plaster from sticking but it makes it easier to separate the plaster from the wood if it does. (If the plaster does stick to the wood just tap it with a hammer to dislodge it.) I don't tap the bucket or the box. I gently create a wave in the plaster mix and let the bubbles rise that way. I like to hold a jiffy mixer in my hand and mix plaster that way. Using just my hand doesn't seem to agitate it enough.

Hey Paula. Palladium is designed to be brushed onto a pot. (As far as I know?) Did they sell you a "dipping" version? As a brushed glaze I presume its got ingredients that lend themselves to ease of brushing. (Not ease of dipping.) As such I think that's where one problem lies. As a brushed glaze Amaco recommends three coats. It sounds like you thinned it too much and the shiny chrome like finish can't develop with a thin application. (Unless you like that color?) I brush on 5 coats and almost always get a nice chrome like quality. I'm firing the glaze to cone 6 on slip cast porcelain. Sometimes I do get blisters and refiring usually smooths them over. I gotta figure 5 gallons is several hundred dollars? I can't afford that but maybe a smaller amount. Shoot me an email and let me know the cost.

There is one "unexpected" when you have multiple glazes sitting around: you find out some glazes, sit well, over time, and some do not. You presume all glazes will be the same, one year later, but you find out, the hard way, that is not always true. Of course, the glaze will probably be thicker and need to be thinned, because you think it's just a water issue, then you glaze a pot and find out the glaze has changed. (Not so much color wise but ease-of-application wise.) Different glaze ingredients absorb water at different rates. A good glaze is one in which the materials absorb water in a uniform way such that the glaze is consistent over time. A bad glaze is one in which one ingredient absorbs water differently and causes the glaze to change over time. Allowing a glaze to sit, over time, is really the only way to know for sure which type of glaze you have. Some glazes don't sit well, over time, but they have an appearance that customers like. In those cases its best to simply make a new batch when a customer orders it. (It's just a question of whether the order is large enough to justify the expense?) "Waterfall Green/brown" is one such glaze. Beautiful color but application/firing issues seem common. Another factor that can play into your decision is your customers preferences and your firing method. Most folks who are glaze firing electric desire to have uniform color results firing to firing. Most folks who are glaze firing gas, natural or propane, expect/desire variation from firing to firing so glazes that change are not altogether a bad thing.

Yes and no. It all depends upon how the model was treated. As Peter suggests "waste molds" are really only intended to be one time objects. However, sometimes they are so labor intensive that you just want to use it to make a test piece. (To see if it works.) Did you have to use lots of mold sealer to make the waste mold? (in other words, is the waste mold really "dirty" from the making process?) There have been many times that I have used a waste mold to make a test casting. However, I accepted the fact that it might not turn out, as expected, and it was "just a test". ps. if the mold parts are "dirty" washing them will help and waiting for them to dry is a good lesson in self-discipline.

It's not so much that it will fall off AFTER the firing as much as it will probably fall off DURING the firing. At which point it will land on your piece in a most undesirable way. Then you have a real annoyance to deal with. If it's a prized piece run a few tests before you commit to any "attachment" method. (Most fail in my experience.) As Oldlady suggests it's probably best to sand down the point of the crack, before glaze firing, and accept that it was not meant to be part of the design. More than likely, you'll receive compliments, either way.

To give nuance to Peter's suggestions: The big key is to stretch the clay in lots of directions while you're rolling it out. Will you be making the slab on a slab roller or doing it free hand? If doing it free hand you will want to use various thicknesses of wood sticks to guide you in reducing the thickness. Plop down sticks that are 1" thick and roll the slab. Then pick up the slab, turn it over, spin it 45 degrees. Then roll it with the 3/4" sticks. Pick it up, turn it over, and spin it 45 degrees again. Then plop down the 1/2" sticks, roll the slab again. Rotating and flipping is the best way to minimize warpage on big slabs. If the clay is too wet you can sometimes stretch it too much at the corners, when you pick it up, so its best to let the clay stiffen a bit between rolls. Firing the slab is another challenge: are the kiln shelves flat? Is it worth it to go out and buy new shelves that are perfectly flat for the project? (Sometimes it is.) All the silica in the world won't help if the kiln shelf is warped. (Yes, a bed of silica can mitigate a warped shelf but a not warped shelf is the better place to start.) Clay intended for throwing is considered "plastic". That usually involves shrinkage. Hand building clays tend not to have so much plastic clays in them. Meaning...less shrinkage. Making a large slab is usually best with a hand building clay. The drywall trick does work well. In place of it several sheets of newspaper, under weight of a flat piece of wood, also reduce warpage in drying.

Hello Christy Ann. Manual kiln sitters can be adjusted to match up with witness cones. It sounds like no one ever took the time to adjust your kiln sitter. There are several small screws that allow for the various moving parts to be adjusted. I won't offer advice until you post a picture of the actual kiln sitter. Bill's advice about a standard firing schedule is a good starting point. The clay body you're using, and your making method, (throwing, hand building, casting) helps us give more accurate firing advice.

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.