Casting

[Laurence Finston]

On another thread, somebody asked about making rounded objects for paper models. Another poster mentioned dapping or doming tools, which I'd never heard of. I don't want to hijack that thread, so I thought I'd start this one.

Of course, having seen the dapping tools, I had to have them, so I ordered some. They came from the U.K. and it all worked out very well. The punches were in double plastic bags and swimming in oil. It took me a couple of hours to get them cleaned off, but the oil hadn't leaked significantly and there wasn't a speck of rust on the punches or the block.

The photos show a small spring form (for baking) and a miniature casserole form (or a family-sized one for very tiny people), filled with wax. I'm using paraffin which I colored with either crayons or candle color or both. It had been sitting in my basement since 1999 but it melted down just fine. Any dust or impurities separate from the wax when it melts and the clean wax can be poured off.

I melted the wax in a bath of boiling water and poured it into the forms. At my first attempt with the spring form, the wax ran out the bottom, either because I failed to fit the bottom in properly or because there's just not a tight seal. For my second attempt, I placed it in a container filled with sand to prevent any wax from running out.

Unfortunately, I didn't think of it this time, but next time I will use a level to ensure that the form is straight.

The green stuff is a kind of modelling material for children made of biomass with food coloring. It had been packed in a box for at least ten years, but amazingly it's still soft. I was afraid the hot air gun would melt it, but it didn't.

I divided the spring form because I've found that it doesn't work well to try to make two impressions in a single block of wax, because the second one will cause the first one to deform, if the wax is too soft and once it's too hard, you can't make the second impression. It's not worth it to try to find the exact correct moment.

The wax was hardened, so I used the hot air gun to melt some of it where I wanted to make the impressions.

The next thing to do is to trim the holes. Then, the plan is to make plaster positives, then negatives from the positives and at last the final positive in papier-mâché.

My first attempt at this a couple of weeks ago failed, because I tried to cast the papier-mâché directly from the wax and it was taking forever for the water from the papier-mâché to evaporate, so I finally just took it out. Wax, of course, repels water, so it was only able to evaporate from the surface of the papier-mâché. Once the plaster has finished giving off water, after a few days, it can absorb and transpire an amazing amount of water, so it should dry the papier-mâché right out.

[Laurence Finston]

I have now cast the plaster positives and that's it for today. I have to wait at least overnight to do anything else with them.

I forgot a couple of things which would have improved the results: 1. I forgot to coat the wax with vaseline before pouring the plaster. There was no wax in the pie pan, so the slab for drying clay came right out. However, the plaster in the casserole didn't and I'd also not thought to make an edge of clay or wax on the top, to make it easier to remove the plaster from the mold. It finally occurred to me to put the casserole in a bath of boiling water to melt the wax. Then it just popped right out.

I got the mold and the cast out of the spring form alright, but I couldn't separate them until I used the hot air gun to melt the wax somewhat.

2. I didn't think to use a brush to apply plaster to the depressions made by the dapping tools before pouring to avoid air bubbles and to make sure that the whole area was covered. In general, the result is not as precise as I had imagined it, however, I think it should be possible to touch up the positive before casting the plaster negative. I'm happy enough with the results to continue, rather than starting over.

3. In the casserole the dapping tool touched the bottom, so the "north pole" (or "south pole") of the dome isn't quite perfect. I knew this before, though, and decided to try to salvage the cast. I think it's good enough to finish, especially since it's a first attempt.

Now, the most important thing, apart from getting my kitchen table back, is to ensure that no plaster is left on the metal tools and equipment. The black coating on the pie pan and the spring form may protect them, but I don't want to bet on it. With normal steel, if you have a tiny speck of plaster on it, the next day you'll find a rust spot underneath it. Otherwise, this operation made kind of a mess, but most of it will keep.

[silverback920]

That's a ton of work, all unnecessary. The dapping block has several hemispherical depressions. Choose the size you need, then select a tool of a slightly smaller diameter. Coat the block and tool with petroleum jelly, then smash-form the paper mache into the block with the tool. When the dome is dry, it'll pop right out of the block. Bash to shape, file to fit, paint to match.

[Laurence Finston]

Thank you for your reply. However,

1. That way, all I would get would be the dome, whereas with the plaster molds, I can also make domes or hemispherical depressions with a margin around them of any shape and thickness, within reason.

2. The block has fewer depressions than the number of punches and they don't all match in size. For instance, for the largest depression in the block, there is no matching punch. Of course, I can use a smaller punch, for that one and any other larger ones, but the main point is that I would be limited to the sizes of the depressions on the block.

3. Most importantly, I would no more leave wet papier-mâché in a steel tool for as long as it would take to dry than I would leave the tool soaking in water.

4. Once the molds are made, positives and negatives, it should be relatively easy to use them to make more out of plaster. Then, I could "mass produce" domes and depressions.

5. If I wanted a solid hemisphere, e.g., in order to have a surface for gluing on the bottom, this probably wouldn't work well, especially for the larger sizes. I mentioned how long it took for the papier-mâché to dry in the wax mold. It wouldn't be any different in the dapping block.

6. I can use the plaster molds to cast domes and depressions in other materials. First of all, plaster, then concrete, wax, lead alloys, acrylic ...

[Laurence Finston]

I've dressed the molds a little with chisels, plaster knives and a couple of engraving tools and I plan to fix them up using spackle before filing and sanding. Supposedly, it's possible to mix up more plaster and use it, but the workpiece needs to be very wet and may need to be soaked. If it was a sculpture, which should only contain known and permanent materials, I might try this, but since it's just a mold, I'd rather just use spackle and not soak the molds. I have my doubts about whether the other method would really work, anyway.

I've put in some more clay to reconstitute. It loses a lot of volume when it dries to working consistency on the plaster slab. The block of dried-out clay is as hard as a rock. When I first broke off some of it the other day, I didn't know where my mallet was, so I used a normal hammer (normal for Germany, not so much for the US). In the meantime, I've found my mallet, which is the right tool for the job and works much better. The head weighs about 100 pounds and all you have to do is lift it and let it fall. It took me less than half the time to break off the pieces and I didn't bash my left hand while doing it.

[Laurence Finston]

I find that tools are often aesthetically pleasing but these 5 are among my favorites. For modelling I will be needing my calipers and dividers, so I got them out.

The only one of these tools that has been in regular use is the flat right angle. The others have been in storage for 25+ years but they've held up
very well despite that.

[Laurence Finston]

The spackle, of course, has dried and hardened, but the plaster is still fairly moist. It will file better in a couple of days, but I touched the molds up as best I could and want to get on with casting the negatives.

If you have any rasps and files you want to ruin, plaster is a good choice. For this reason, I have separate sets of them for wood and plaster. Also for chisels, which is a good idea, but not strictly speaking necessary, since chisels can be ground, if need be, and sharpened. I have a set of chisels for plaster of exactly the same brands as I have for wood, which is a luxury, but "man gönnt sich ja sonst nichts", as we say in German. (Approx.: "One doesn't give oneself any other treats.")

The casserole dish is tapered from the top to the bottom and the mold was upside-down at the top, so I had to file it down to make it fit into the bottom of the dish. I also made it thinner so the bottom of the negative would be thicker. The surform file is a good choice for this task, since it can't clog and removes a lot of material quickly. It can be hard to control, so I tend not to use it for detailling but for this purpose it's perfect. Also, the consistency of the plaster at this point is good for this. In a couple of days, it would be much harder.

[Laurence Finston]

I filled up the bases of the casserole and the spring form with wax and made a wax edge at the top of the casserole. In the past, I used clay for this purpose, but wax is easier to handle and makes less of a mess. It also works well to melt it when it's time to remove the cast from the mold, as I discovered before.

This time, I remembered to grease the forms and the models and to use a brush to apply the plaster to the "business" part of the mold. I didn't have enough left of the ordinary modelling plaster I had been using, so I opened the package of alabaster plaster that I'd ordered recently. It was noticeable that it's finer when I was sprinkling it into the water.

Now, I will give the casts a half hour or so to cure and then remove them from the molds. With a little luck, I should be able to try making the first papier-mâché cast this evening.

[Laurence Finston]

Casting is like playing the French horn: You know what you put in, but heaven only knows what will come out.

(See photos with next posting. I messed this up somehow.)

I'm happy with the result, especially since it was a first attempt. The molds aren't perfect, but they are usable as prototypes and I think I can get better results using basically the same method.

To get really precise results, I think one would need to use hardwood, probably with a lathe, precision tools and a casting box with sand. Not within my budget at the present time, I'm afraid. However, that degree of precision may not be necessary and if it is, then I'm out of luck.

Next time, I will try using clay instead of wax for the impression. I think this will work better, since you have more time to work and can add or remove clay as needed.

I forgot to divide the top of the round mold but that's not a big deal. It would be no problem to saw it, but I may not bother. I might even do the same thing again, but with more depressions.

The alabaster plaster is significantly harder than the ordinary plaster, even though the former is fresh and the latter was cast yesterday.

I have to do something else today, but I hope to try the molds with the papier-mâché later on. I will try touching up the depressions but I really need to wait a few days for the molds to harden in order to do a proper job.

[Laurence Finston]

More photos.