I've done enough casts now (10) to build the bridge from the Hirst Arts bridge mold (#74) and I am letting them dry out before assembling the bridge (which is my goal for tonight), but I noticed that in spite of my most diligent efforts to follow all of the instructions on the Hirst site, I still ended up with a few bubbles.
Also, I ended up with a lot of excess and spilled plaster, which is sort of a mess to clean up.
So last night as I was going to sleep I pondered ways I could improve the plaster pouring process to avoid waste, spillage and bubbles.
My brainstorm was to use a syringe to have absolute maximum control of the process of filling the mold cavities with plaster. I figured this would work to improve all three areas.
I just happened to have a syringe lying around (it came with a guitar humidifier device that I no longer use) which had a large enough nozzle to apply plaster to the molds cavities. So this morning I did a test cast using the syringe.
It may have taken a minute or so longer to use the syringe due to having to refill it from time to time, but it wasn't a major increase in time invested.
But it was a huge improvement in controlling the waste and spillage. In fact there was virtually no spillage whatsoever, and I may have ended up with a teaspoon of wasted plaster (which I scooped up and reused in a large mold anyway). But the best news was that when I demolded the items, no bubbles.
So I'm wondering if anyone else has used a technique like this? Or some other similar technique. I was very pleased with the results. The only shortcoming of the current approach is that the syringe I'm using is very small and has a blunt and short nozzle. I am going to get one with a longer and curved nozzle which will hold more plaster to reduce the number of refills while filling molds.
I lucked out when I started using Hirst molds... my dentist is into making shadowboxes, so she helped me out with a vibrating table to work the bubbles out.
I've found that using some Jet Dry to coat the molds prior to casting is the single most useful thing... submerge the molds in your water/Jet Dry mix, and work all the bubbles off the mold. Once you have no air bubbles, take them out, drain them, cast... then put them on your pounding board/vibrating table.
I use a plaster (Excalibur) that shrinks a bit when it dries, so I end up over-filling the molds and have to scrape a bit once they set, but I have a few small 'rocky' molds I throw all my scrap plaster in.
What type of plaster are you using? I'm not sure how my Excalibur would do in a syringe, but that sounds like an excellent method when I'm not filling 5-6 molds at a time, but want specific pieces cast.
Only one batch of mine has had any issues with bubbles, (was too thick, I didn't prep the mold at all, and I did not shake it soon enough) and with plaster being dirt cheap I haven't really cared about spillage. I would rather overfill and make sure my blocks are the right height. I'm suprized the syringe was able to work with plaster, and would think that it would be more expensive to replace than the plaster.
Are you using a suffocant?
I am using Excalibur too (although I do have some standard plaster of paris and some Lightweight Hydrocal I use from time to time).
I have thought about making a vibrating table. Our dog did a number on a massaging pillow and the vibrating element still works (it's just a big lump of plastic that you put a couple D batteries in) so I could just screw that to a small table...
I use the "wet water" recommended by the Hirst "Tips and Tricks" page. I use a few drops of dish soap in a small spray bottle filled with water and I spray the mold and then spin it and tap it on all sides to (hopefully) get the whole thing moist. Then I pound out the excess wet water. That works well too.
I'm going through my 25 pound bag of Excalibur much faster than I had anticipated... and it wasn't cheap (roughly $50 when you add in shipping). So spillage and waste is an issue for me, at least with this stuff.
But it's amazingly strong. And it paints well, although it does suck up a ton of paint when you put on the base coat...
edit: Oh, and the syringe isn't wasted. It's totally reusable. I just suck up a bit of water when I'm done with the plaster and squirt it out, it works fine the next time.
Oh, and while I'm on the subject...
How do you dry your cast Excalibur objects? Do you bake them in the oven, or just let them air dry.
I've been letting mine air dry for almost 24 hours now, and I had a small space heater blowing on them for about two hours last night.
How long should I wait before building the actual item?
As others have mentioned, jet dry works pretty well. Hirst Arts has a detailed "wet water" method on their website.
I place the mold (or two/three) on a piece of wood, fill them, and then bang the heck out of the wood. I also do this in an outdoor area, because it is messy.
As for painting and gluing, definitely wait a while. I usually spend a day casting, and then I'll work on building the next day at the earliest.
Unless I'm having major bubble issues, I really don't care about it. Bubbles become places where an adventure stuck a piton, or there was some erosion, or it's where some common spiders live. I just work with it.
I let my Excalibur air-dry for 24 hours. It's actually a chemical reaction, not simple drying, and while you can accelerate it a bit with heat, I usually just cast stuff and set it aside to use for building later. IIRC, people on the HA boards recommend waiting 24 hours before gluing because the curing process does give off a tiny amount of heat that could interfere with glue (perhaps because of the heat, perhaps because of expansion and contraction).
Thanks, the first batch I'll work with will have been drying for a bit more than 24 hours when I start assembling them. Plus two hours of that was with a space heater blowing warm air on it, and five hours was with a fan blowing on it. When I touch them, they don't feel cold anymore.
I've done four casts now with the syringe and it is really working great.
I am a perfectionist, I'm not even sure most people would care about the bubbles I am trying to get rid of. And it's only on a few blocks... but as I said, I'm a perfectionist...
By this time tomorrow I should have the bridge built and all the blocks cast for the tower. I'll probably have to wait until Sunday to build the tower unless I get impatient (and I probably will) and end up drying the blocks in the oven...
I'm going through my 25 pound bag of Excalibur much faster than I had anticipated... and it wasn't cheap (roughly $50 when you add in shipping). So spillage and waste is an issue for me, at least with this stuff.
Talk to your dentist, and ask if you can order dental plaster through them. They almost certainly get it both cheaper and with less shipping costs than you do, and if you're on good terms with them, they shouldn't mind adding a box onto their order.
My dentist doesn't use Excalibur herself, but she can get it through her distributor, and she gets it shipped free... so, while it's a 'YMMV' deal, it's worth looking into.