Jeff Longtin

That's a pretty deep crack. Hard to say exactly how the plaster will react in the future but it certainly doesn't look good. The crack appears to undermine the wood plate so I'd be hesitant to use it to anchor a bracket. Back in the 90's I made molds for a gift shop that made and sold cement figurines. They also cast table bases made of cement. They would roto-cast those. Previously I'd never heard of roto-casting. There are certain cements made for casting figurines/sculptures. Have you tried any of those? All of which is to say I would recommend you not solid cast this/these. Mix a small amount of plaster/cement and brush it on the inside of the piece. After 5 - 10 minutes apply another coat. Maybe do a third coat. Then, after it completely dries, you could GLUE a bracket into the cavity. Lots of epoxies are super strong and with a hollow cast you have less weight to worry about. Good luck.

I would abandon the cookie cutter process immediately. (Adding too much stress to the clay.) Maybe use a template and fettling knife instead. A less plastic clay would also be a good start. Drying between sheetrock/drywall is good. What kind of slab roller are you using? One with a single roller or two? If it has a single roller you might want to flip the slab between compressions. A double roller isn't a guarantee of no warp but it helps. Beyond that welcome to the world of slabs. You would think it's easy but it can be way more complicated than throwing.

Having recently worked at a pottery shop, where inexperienced young people mixed 10-20 gallons of glaze a day, I'll offer this: more than likely you carried over one of the percentages. Rather than use 30% Spodumene you may have used 50%, for instance. Considering the desired glaze is extremely matte I would suggest you added 20% Bentonite rather than 2%.

Welcome to the Forum Carol. Do we have any idea what the mask is made of? Short answer to your question: more than likely some damage MAY occur. (Especially if you have no idea what the antique is made of.) Alginate is probably the least harmful but it is water based so the piece may see some water damage. Urethane rubber removes the water element but the piece would need to be sealed with a silicone like substance which may affect the piece. A photo would also help.

Min has picked up on what I was suggesting. If you fire a cone form on TOP of a cone form you will create a stressful situation, for the clay, as it shrinks in the firing. (The clay will shrink around the cone and probably crack as a result.) If you fire a cone shape with the cone pointing downward, in a setter that is also a downward cone, a bowl basically, then the cone will only sink deeper into the cone as it shrinks. As there is nothing inside the cone you are not putting stress on the clay. The likely hood is greater that it will retain its cone shape this way. Leaving the middle of the cone support open allows the clay to shrink and sink without interference. If there is no hole then you run the risk of the cone bottoming out in the middle of the setter.

Separate the two processes: 1) filling the seams/gaps, and 2) filling the mold. 1) pour a small amount of slip into the mold and use a brush to push the slip into the seams/gaps. Wait a minute or two. Use the brush a second time to push the slip into the seam/gaps. Wait another minute or two. Do the same thing a third time if necessary. Wait another minute or two. After you see the seam/gaps sufficiently filled THEN go ahead and start filling the mold. Go slowly to make sure your gaps are filled and no slip is leaking out. If it looks like you filled the gaps sit back and breathe a sigh of relief. Right now my purse vase molds are just plaster parts held together with straps and rubber bands. The shape, 8" wide, 8" tall, 3" deep, is really susceptible to blowout if I forget to seal the seams. The above method assures that the slips stays in place and I get a decent casting without a lot of mess or stress.

Have you thought of firing the piece upside down? (for the bisque anyway) Build up a mound of sand, as Callie suggests, and then push the upside down cone shape into it. It won't help you in the glaze firing but at least you can reduce distortion in the bisque.

As best as I can tell here's what's happening in this situation... when the liquid slip first makes contact with the dry plaster surface it IMMEDIATELY dries. Then, as more slip continues to flow over it, it forms a "ridge". In theory you have two solutions: as I mentioned previously, you can make the point of contact an area not visible from the top OR you can reduce the degree, to which the plaster dries the slip, at that first point of contact. You would do this by wetting the mold before you start casting. (Simply hold the mold, under running water, for just a second.) Wetting the mold may reduce the height of the bump but it will also slow the casting process. Therein you have a choice. Porcelain doll artists first introduced me to the idea of wetting a mold before casting. (To reduce sticking.) Subsequently I found it has other benefits as well.

I've read the post a few times and I'm still confused. Long story short...you have a slight ridge in the bottom of your cast piece and it bothers you? Have you tried using a metal scraper to remove the ridge after the clay has set up a bit? It's not something I've really been aware of these days but I do think its an unavoidable part of the process. If you can start the pour in a part of the form where the bump is less noticeable that would make it less effort for you. The other option, which I've employed on some forms, is to reverse cast the form. Have the mold form the INSIDE of the piece. A bit more challenging but it removes the ridge issue.

When I make a plaster form it usually takes 5-7 days before its completely dry. I place plaster forms on plastic grates that allow air to freely flow around the object. If I forget to prop the piece properly and leave it on a wood board, or plastic sheet, it will develop a brown/green mold where it was in contact with the wood or plastic. "Brushing off" - unfortunately this is subjective. When you pick up a plaster mold/form you will usually have some white powder on your hands/fingers. An indication that the plaster form is deteriorating would include an increasing roughness to the plaster surface.

Alginate is a skin safe molding substance. Mixed with water it has a consistency like pancake batter. Once it sets its very much like set jello. It's firm for a day or two but after that it starts to shrink and crack. Because of that you want to get a plaster casting made a s soon as possible. I haven't used it in ages but I don't think it discolors plaster.

I'm sorry for your loss CharliesMomma. In 2000 I was commissioned, by a funeral home, to make a death mask of a 2 yr olds face. He was cremated the next day. It was a very emotional experience for me and the woman who assisted me. (She too was a mom.) I then turned the plaster casting into a porcelain deathmask. The little boy had four siblings. Each one received a casting. When the four kids, and mom and dad, came up to get the castings the mom gave the money to the youngest child, a young girl, and she handed me the cash. That was a very unusual experience. The suggestions so far are good. Its hard to say why the pieces are discoloring. Were they wrapped in newspaper or some other material that might have ink on them? (Wet plaster stains easily.) At this point I would suggest you simply let them dry completely and see how they look. More than likely the discoloration won't cause them to deteroriate. It will simply be a discoloration.

I hate to say it but I would suggest you abandon this project. It doesn't look like the molds are friendly to the plaster. See where all the plaster remains in the mold? Those are undercuts. In the casting process those will be little bits of clay that need to be cleaned with a fettling knife. That's time consuming. Do you like the surface texture resulting from the 3D printing process? If not I would suggest you use the plaster molds you just made and make plaster block molds with them. Now you have a plaster mold that can be easily sanded. Use this new plaster block mold as your new mold. Ten years ago I made urethane rubber block and case molds, from a plaster mold, and they have beautiful smooth surfaces and they flex just enough for new plaster castings to pop out easily. They make mold making super simple and the molds come out perfect. (Because I took time to make the plaster originals perfect as well.) Good Luck otherwise. I appreciate the effort.

The key to Murphy's is to create a lather. (with a brush) Then wiping away the excess lather with a sponge, then wiping away excess bubbles with a dry brush. Do this three times and you'll have a nice "oil-like" surface. (Which flakes off/dissolves in a few days.) My concern with pouring plaster against wet Murphy's is that the wet substance MAY absorb into the plaster surface. You might have a different experience, however? Another option is to use a food safe silicone spray. In the US there is a product called PAM. In another mold making forum this was mentioned as an effective release material when pouring plaster onto nonporous surfaces.

Great response Min! A local artist thought she could get away with 15% Mason Stain Black, 6600, but once she put it into production she found out, soon enough, that 15 was a nightmare. I find 2% best. (It only creates a dark gray, however.) I'm adding it to cone 6 porcelain slip. a note: I find a curious thing when testing colors in porcelain casting slip...the initial tests will often come out beautiful, the color looks great and the clay body performs well. It's only after the slip has sat for a week that the problems show up. For some reasons many stains, in high proportions, will cause a slip to flocculate. (Thicken up in other words.) I find iron-based stains cause a porcelain slip to gel at 5% but not so much at 4%, for instance.

Having worked in a pottery shop, where lots of young people are convinced that one day in a pottery shop exposes one to a lifetime of silicosis, I'll offer this thought: Did you see the interview with the "industrial hygienist"? Did you see how cluttered his office is? Do you think a cluttered office is cleaned regularly? I think the hygienist might want to clean his office before he's interviewed on tv again. That said, I had no idea "glacier silt" is actually a thing. (I presume its a form of salt?) Makes me wonder if "road salt" used all over the US is also creating a similar problem? I appreciate the posting.

It looks like the top ring has no elements in it. Is that correct? Thats known as a "blank ring". Does your kiln have four layers of brick and do they all have elements?

I've not used blue spark but I have used Palladium quite a bit. I apply 5 coats. (to a 06 bisque pot, porcelain) The first two coats are thin. The remaining three coats are progressively thicker. I only use bottle thick as the final coat. I'm not used to brushing glazes so the bottle thick always scares me. (Along the lines of what Dick mentions.) Bottle thick always seems to leave brush strokes/pinholes so I find the two thin layers prevents that from happening.

Hello Sue. Either side will dry the clay equally well. The smooth side is simply easier to clean so I would use that side. The next time you pour you might take a moment to watch the plaster set. At the 10 - 15 minute point the plaster will lose its water surface and start hardening. If you catch it in time you can use a straight edge scraper and lightly scrap the surface of the plaster. If you do this several times you will slowly develop a smooth surface. Its an acquired skill but one I use alot so that my textured plaster forms can easy sit on level surfaces. (Which reduces that chance they might warp as they dry.)

Yes, I've done that before. I just added a small amount of water. If there appears to be solids I've screened those out as well.

Mark makes a good point. Glue is for broken molds, two pieces, not "cracked". Cracks, on the inside of the mold, can sometimes be filled with a thin slurry of plaster, but sometimes not. It's usually best to apply the plaster in a water like consistency and build it up in layers. This repaired crack will likely be a spot of sticking, for the cast piece, but after several castings the sticking should decrease.

Hello Zan. When I was eager to make molds I left my name with several local pottery shops in the area. (Minneapolis/St Paul) That generated regular customers in the 90's. You might connect with the local pottery supply shops in your area as well. Good luck

Heating a mold - so you know, 120 degrees is the highest temperature you should expose plaster too. (Beyond that it starts to break down.) The cracks could indicate a problem that simply gets worse, and a new form needs to be made, or you may get lucky, and they don't increase? I find Elmers glue to work well to temporarily repair cracked molds. (Elmer's is a simple, white, water soluble glue, widely available in the US.) However, once the plaster becomes sufficiently wet, the glue sometimes loses its bonding ability and the pieces separate again.

Murphy's Oil Soap is an effective way to seal plaster, temporarily. Depending upon how it is applied it will sometimes just flake off the plaster surface after the plaster has dried. (No need for washing, in other words.) Murphy's, like all soaps, develops a lattice structure only after it is lathered up and then allowed to dry. (After the suds/bubbles have been wiped away.) The drying creates the lattice structure that seals the surface. If one is making a mold of a fired ceramic object, Murphy's doesn't really seal the surface. It sits on top. If the plaster is poured while the Murphy's is still wet it may absorb into the freshly poured surface. If that happens the plaster may become weakened and may erode sooner than normal. If I'm making a mold of something, other than plaster, and I'm unsure if it's completely nonporous, I will spray acrylic paint on the object. (That way I know the surface is sealed.) In mold making, like all things creative, there are "rules" that can make the process more effective. On the other hand, like all things creative, one can break the rules and still have successful outcomes.

Hello Henrietta, Welcome to the Forum. My suggestion is that you should not wait 5 minutes for the plaster to "get wet". Two minutes is all I leave it for. (Both USG and Georgia Pacific recommend the same.) You really just want the particles to get moistened. The key to plaster strength is the mixing. Shorten the wet time from 5 minutes to 2 and make up the difference in mixing. That way you can pour while the mix is still fluid but you also have plenty of mix time. Good luck!