Jeff Longtin

Hello JJ, In my experience reclaimed casting slip is not like new casting slip. Castings come out thicker and they take more time to set up. (The clay also tends to stay soft much longer.) As a result I try to mix my reclaim with new whenever possible. (50-50 isn't bad but 25% reclaim and 75% new is much better.) Trying to cast pots with reclaim slip and expecting it, to be like new slip, only leads to frustration. (Been there done that.) Knowing the kind of clay you have would help greatly. All three clays, porcelain, stoneware, and earthenware, need to be deflocculated, but they can be deflocculated with different substances. Darvan 7 is common for porcelain, but sometimes sodium silicate is still used when manufacturers want to save money. Soda ash can also be used. Generally speaking I don't add deflocculant to reclaim. (As its already in there.) But different clays respond differently. Your specific gravity at 1.68 is good. It indicates you should have enough water in your slip. However the gel state could indicate a need for more deflocculat but before I suggest that more info would help.

Are you aware that you can reset the temp at which your board overheats and you get an error code? I forget the exact path but its super simple to adjust. I think the factory setting is 165 but you can raise it a bit higher.

Hello Startile333, Welcome to the forum. Your proportions are good. The last few batches of Ceramical, that I've been using, have mixed thick. When that happens I add a tad more water to get a nice smooth mix. (At times the surface of the mixing Ceramical was a sea of bubbles. Some breaking, some not. Adding a touch more water allowed the bubbles to break more easily. Before the pour.) Repair - white Elmers has worked best for me. I usually have to wait for the glue to become tacky before I get a good bond. As far the problem is concerned...it's hard for me to see what the problem is, exactly? I see the holes...I don't see the context?

Betty, I found the same thing, none of the Amaco Black LUG/Velvet underglazes are friendly to clear glaze at cone 6. To increase flow I add a little Zinc-free clear , to the underglaze, and that seems to lessen pinholes. (I forget why I picked zinc-free but it seems to work.)

As a general rule its best to store a mold assembled rather than disassembled. (If its a multi part mold.) Mark's comment is a good one as well. Like Chilly suggests wiping the mold down with clean water is best. Do not store molds near a heat source if you don't know how hot it gets. Above 120 degrees and the mold may start to break down. Metal racks made of wire are great for storing molds. They allow for air movement. Shelves made of sheet metal are fine if you know the mold is completely dry. Same with plywood shelves.

A few years ago I had just completed a Ceramical press mold when I brought it out and saw several previously made molds with a light brown staining. As I looked at them in surprise the young gal pressing, at the time, apologized and said she spilled coffee on a few molds. At first it seemed it was just a staining of coffee and nothing more. It was only a few days later, when another person tried to use a stained mold, that the deterioration became evident. I had to remake the mold as a result. Later that week I poured some black tea onto a pottery plaster mold, in my studio. At first no damage, but after a few more days, the pottery plaster was soft. Not really sure why it happened but I do try to keep beverages away from molds as a result.

At one point books, served my needs, but not much anymore. These days its mostly google and internet searches. This forum often has great information as does a previous forum I participated in, Clayart.org. The environment was a bit more combative but good information nonetheless. One aspect to Clayart that is interesting/helpful is that much of the posts were made in the 90's and 00's. Back before kilns were programmed and colors were stains. In that regard it enables one to understand a chemical reaction/glaze result from a different perspective. Digitalfire is one such website that provides lots of deep information about many things ceramic.

Some additional thoughts: There is a website called "artmolds.com" that has all sorts of mold making products. I first became aware of it when I was working on a deathmask project. Artmolds has several videos and kits in the area of "lifecasting". (Maybe everyone could make a cast of their hands?) Another thought: find a ceramic mold website and see what they have to offer. Perhaps order a bunch of molds that you think the kids might like to work with or ask the kids to make their own choices. Then buy some premade casting slip and have the kids use the molds to cast up pieces. This way they're working with casting slip and they're working with plaster molds. You can then use this as a barometer to see which kids really like the process and which ones don't. In my years of making molds I have never met a person who fully understood the complexity of designing/making a mold. Even the most experienced clay folks seem to find the concept difficult. Expecting young people to understand it may be asking too much. (at least initially.) Make it easy for them initially, to build their enthusiasm, and then build on that.

Welcome to the Forum Artisunteacher. Its a broad question with many answers. I don't think there are any "kits" out there that you could purchase. (The only kit I know of might be for casting hands or feet of newborns.) One way you might approach it is to have your students bring in something they would like to be molded and YOU make the mold. (Have a Casting Day/Party type of thing.) They could then use the resulting molds to make items that they desire. That way you might stimulate their enthusiasm, for using molds, but avoid any mold making disasters that might occur if they do the casting themselves.

I'll pick up on what Hyn Patty says: some of us started making molds before the internet and we really only learned from our mistakes. In theory plaster is inert and should behave the way we expect it to. Unfortunately that is not always the case and you need to improvise. That's when new ideas are born and new methods created. Continue your trials and feel free to post questions when they arise.

While Min and Neil address the firing issue I'll ask this: are we talking boxes of slip that have been sitting around for over a year? Did the slip thicken during that period? Was the slip exposed to the elements as well? (Perhaps the slip became contaminated?)

Hello Shane, welcome to the Forum. My guess is that a liquid was spilled onto the molds. Perhaps coffee or an acidic cola? In the 70's my late father worked for a large advertising agency that was in the process of opening offices around the world. (BBDO) One of their more successful locations was in Sydney. He visited several times. He always came back with cans of Fosters beer. The opera house had just been built so he also came back with postcards of it. He enjoyed his visits to Australia.

Hello Ida, Using plasticine isn't bad but some varieties soften with heat, from the setting plaster, and this can make it difficult to pour the second mold part. If you want to post a picture I can offer more thoughts.

Hello Handy, Not quite sure why this is happening to you. When the "unexpected" happens to me I just roll with it. Rather than try to figure out why this is happening I would simply use this first casting as a "throw away" mold and continue pouring the mold. When you pour the second half it will likely go beyond half way and may hold onto the piece. In this case I would take a small trimming tool and carve back the plaster. (to the line you indicated/prefer) Generally speaking I do not pour molds off clay. (as the embedding material) If the form is complex I may use clay for that first pour but once plaster is poured I use that as the embedding material. (And discard the piece that was poured against clay.) This way I can more easily control the parting line and create a mold with nice tight seams.

I'm not a chemist but it seems the salts move through the mold and accumulate on the surface. Washing the plaster might help but sanding the surface, and actually removing the salty buildup, would be more beneficial. (People who slip cast a lot have told me this.) Drywall sanding screen is a wonderful way to take plaster down easily and quickly. (Though it is kind of messy.)

Interesting fact: once a plaster form has thoroughly dried, after the initial pouring, re wetting it, and allowing it to dry a second time, will completely open the pores contained within. This is probably inconvenient, regarding a wedging table surface, but it would enable you to take the utmost advantage of the plasters aborbing capabilities. The degree to which the plaster slab no longer can absorb water is dependent upon the clay you use. If your clay contains soluble salts, sodium silicate or barium carbonate, for instance, it may slow down with age, but most likely your clay doesn't so you can expect many years of use. As for drying a plaster form: heat is good but air movement is better. Too much heat, above 120 for instance, can start the breakdown process, too much air movement, has no bad impact at all. Put a fan on your plaster slabs and they should dry much sooner than simply letting the still air dry them.

Metallic Glazes - Amaco makes two Potters Choice glazes that I have used. Gold Lustre - which looks like bronze metal, but not gold, unfortunately, and Palladium - this actually looks like Palladium/chrome when it fires successfully. It is not food safe but it is safe for you to use in the studio. (though firing in a well ventilated area is best.)

Interesting tests. (I didn't see this at first, thank you Min!) What does jump out at me is the comment, "It's not any colder than..." By this can I presume that your studio is on the chilly side? (My historic low was 55 degrees at a studio, in a very poor part of town, many years ago.) While I've never really thought "room temperature" had any bearing I do wonder what your actual temp is? Also the state of the molds. Are they cold as well? Could the state of the molds, and the slip, be contributing to this problem? My only other suggestion would be to take individual components and mix up small batches and leave them for a few days in the studio. Then, in an oversaturated liquid state, pour them out onto a plaster surface and see if any offer a similar reaction. You might get lucky and the offending party is obvious or maybe not?

I've been looking at lots of thermocouples this week. We have 18 kilns and I thought the prev kiln person was checking thermo's but it appears he was not. The Skutt's are easy enough to see the L&L Davinci's are less so. (Enclosed ceramic tube.) So I spent some time pulling out tubes and inspecting things. While your photos are good its hard to tell the true state of your thermo. Bill's pictures show the extreme end of thermo wear. The kiln manufacturers suggest replacing thermo's after 60 firings. In our case thats about 3 months. (At the pottery shop new kilns become "glaze kilns" and are fired, every day, to glaze temps.) We only honor the 3 month rule when the kiln is firing slowly or giving odd readings. Otherwise 4-5 months isn't bad. When the thermo has reached the stage pictured it's definitely time to replace. Another example of an old thermo is a bloated end. Rather than seeing two rods welded at the end it appears to be two bloated wires side by side. (With a bloated/bulbous tip.) This was the case this week. I saw lots of bloated tips. While the thermo works the kilns are taking longer than expected and some of them are seeing greater-than-normal temperature variations. Additionally, Bill's other picture shows a thermo as they typically are set. Straight, with a ceramic block that holds the wires and the thermo together. I've not seen a thermo bent, as yours are. When you replace it you might consider replacing your block as well. I'm not a kiln expert but maintaining straightness might not be a bad thing?

Interesting body of work. Thank you for bringing it to our attention. (It always fascinates me to see how potters elsewhere present their work.) Having looked at the sites I don't see transparent glazes. I see unglazed pots and pots with matte glazes. (You know the pot has a glaze because it has spots. The spots are iron bleeding through the glaze.) If you fire an unglazed pot hot enough the clay will start to melt/vitrify and give the appearance of a glaze. I work at a pottery shop (800 pots a day) where we tried to achieve the same effect. We even commissioned glaze companies to make a formula for us. No such luck. Translucent/cloudy, yes, but not transparent.

I suspected as much. With bisque clay as a back drop the method can produce really fine lines. With unfired clay as the back drop you will probably create lines that are much more irregular. (owing to the texture, and nature, of the clay) My method requires you to fire the piece before any other decoration is applied, to burn off the wax resist, but if its the effect you want its not a huge inconvenience. You just need to be careful about how forcefully you apply the needle tool because the unfired clay can gouge easily.

"Scratch through wax..." I'm curious what this entailed? What kind of wax were you using and what surface did you do it on? When I tried the STW method I found the lines very uneven so I abandoned the idea. I didn't have the means to melt wax so I used wax resist. Did you do it on a bone dry surface or a bisque surface? My most uneven results were when I applied wax resist to white underglaze on the surface of a bone dry pot. Sometimes the underglaze would chip, rather than simpy scratch, and that was very uneven. Sometimes the dark fill color would seep under the wax and stain the white underglaze. I thought I could use a damp sponge and wipe away the excess color that didn't stay in the scratch. To my surpise the wax resist simply washed away and I ended up smudging the whole thing in a very unsightly manner. Wasn't the effect I wanted but maybe more what you're looking for?

I would say this is a good place to start. Once I let my mugs sit overnight the problem went away. I think the problem with expecting them to dry, during the firing, is that they get too hot, too soon. Allowing them to dry, outside of the kiln, allows them to dry completely before heat enters the picture.

Another aspect you might look at is temperature. Is this happening in specific areas in the kiln? You might drop a few cone packs into your kiln to see if there are hot spots. If the clay body starts to vitrify, before the glaze has melted, the glaze will "run off" (crawl). Most likey an application issue but over firing can sometimes be the culprit as well. Also, have you allowed the pots to dry thoroughly before firing? This happens to me a lot, with my slip cast mugs, if I fire before the glaze has dried.

An alternative solution: if thinning the slip doesn't help you might consider making the pour hole larger OR adding a second pour hole altogether. Years ago I made a mold of a Vanhool bus model. (I also made the model.) The mold was about 22" long and 8" square. I put one pour hole, at each end, to prevent a vacuum. Wouldn't you know the customer emptied it sideways and created a vacuum. When they called to complain I explained that the two pour holes were intended to prevent such a thing. They changed their method and all was well.