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Saturday, February 20, 2010

For my birthday I got ..

  • an SMD soldering/rework station
  • a 1 litre bottle of good quality Bourbon whiskey
  • an ice-cream cake with a chocolate/ganache penguin
  • a ton of best wishes.

Fourty-four isn't so bad after all :)

Half the cake is left, all of the bourbon and everything is looking like it needs a bit of soldering, desoldering or rework ... now to get to work!

Hand Casting using Alginate

As part of a larger project I decided that I needed a plaster cast of a hand.

I'd read a number of articles about body-part casting using a magic substance called Alginate. This stuff comes as a non-toxic dry powder which when mixed with water turns into a fast setting gel. When mixed it smells vaguely citrus-like. It's the stuff that dentists use for doing moulds of your teeth if you've had any of that sort of dental work done.

I hunted around and found two good sources of the stuff in Australia. The first was just up the street from where I live in NSW at Barnes and the second was in Victoria from Dalchem. Both seem to have reasonable prices and both deliver. Due to my deciding to do this over the christmas/new year period when shops were closed and a mixup with the courier I ended up ordering it from both. Not to worry, I have plenty now.

The basic ingredients for the hand casting are:

  • A large-enough container to use to hold the moulding compound
  • Alginate
  • Plaster
  • A stirring implement or two
  • A body-part
  • Vaseline
  • Water
  • Mixing bowls
  • Paper towel to clean up with afterwards

You can see that I used a 2 litre juice bottle with the top removed to contain the goop. Unless you build a fancy container that you can take apart you'll probably want to use something disposable, as getting the gel out once it has set is much easier if you destroy the container first.

The next step involves preparing the body-part. The advice that I'd read on the net suggested that using petroleum jelly to coat the body-part helps when trying to get the part out of the gel later. This seems to be true, so we'll do that next time too. We used just enough to coat the skin, but not so much that there was any significant residue.

Mary lent a hand here.

Now is a good time for your body-part model to practice the desired posture for the mould. The Alginate mixes into an opaque gel so your model won't be able to see their body-part once immersed, so a bit of practice in adopting the right posture and then putting it into the mould container helps get it right at show time.

Next you mix the Alginate gel. There was a trick here that wasn't obvious until it came time to actually do it. The trick is determing how much of the Alginate powder and water you need. The stuff is reasonably expensive so you want to minimise waste. At the same time the stuff sets pretty quickly so you don't really get time for a second chance. The trick is to work out how much you need and to mix that amount.

The Alginate I used recommended a ratio of about 4.25 parts water to 1 part Alginate by weight. The trick here is what you're looking for is a target volume and I couldn't find any guidance on how you did that.

I started by looking at the container and figuring out what my target volume would be. A simple experiment involving filling the container to the brim with water and putting Mary's hand into it to see how much water was displaced determined the volume of her hand. Armed with that knowledge I figured that I needed about 1.5 litres of Alginate goop to do the casting. I then started with 1.5 litres of water and working out what amount of Alginate I'd need for that.

The result was an enormous-looking amount (by volume) of powder. Looking at the two it seemed clear that there was no way those two were going to mix into an amount that wouldn't be more than what I wanted, even after taking into account how much the powder would pack down. So I scaled the amount down and ended up with about 1 litre of water and about 250 grams of Alginate. It still seemed like a lot, but I trusted my instinct.

Here I learnt something; my instinct was wrong. The alginate virtually disappears into the water. Next time I'll just do the calculations assuming the volume of water pretty closely approximates the desired volume of the gel. As I mentioned earlier there isn't really time for second chances so I ploughed on ahead anyway.

It's important that you find somewhere comfortable for the owner of your body-part to sit. The Alginate comes in a range of setting times. I chose a medium rate gel that sets in about five minutes. That can be a long time to wait while holding a limb immobile in goop. You can see here the result of my miscalculation in volume. I'd intended to cast the wrist as well, but I didn't end up with quite enough to make it.

Now the fun bit. The body-part model slowly eases their body-part into the goop. It'll be cold and slimy initially. A bit of a wiggle of the body-part helps to ensure that the Alginate gel has made contact all over. They now must keep the part pretty much immobile while the gel sets. I insisted on waiting the full five minutes before testing if the gel had set. They'll know when setting occurs, if they very gently try tiny wiggles they'll feel the gel has solidified around their part. This stuff ends up with a texture/consistency somewhat like Silicon sealant. It's firm and rubbery.

I'd read a fair bit of advice about how to actually remove the body-part from the gel after it has set. The problem to overcome is atmospheric pressure and vacuum. The trick is to get air into and around the skin so that the vacuum doesn't hold it in. I've seen people suggesting blowing air in and this might be a good idea. We opted for a gentle pulling motion combined with gentle wiggles.

When the body-part is removed you have your mould. The Alginate capture quite a bit of detail and I found myself somewhat fascinated by it. Apparently it's a kelp extract. You can at this stage rinse the mould out with water if you wish. I didn't bother, it looked pretty clean to me.

Now comes time for the cast itself.

This bit is pretty straight-forward but in some respects is the most difficult to get right. Mix the plaster as per the instructions that came with it. The plaster is quite cheap so I wasn't anywhere near as concerned about wastage as I was for the Alginate. Additionally, it sets slowly so you've got time to adjust your volume anyway. I knew the volume of Mary's hand from my previous measurement so I knew roughly how much I needed.

I factored in some extra to use as over-pour because I wanted the hand to have a solid base, why will become obvious later.

I learned a few valuable lessons here. Firstly, don't rush this bit. I was excited and wanted to see how this would turn out so I didn't stir the plaster properly. I stirred it by hand, next time I think I'll use a paint stirrer and an electric drill. Secondly don't make it too thick. The plaster needs to be fluid enough to reach the extremities of the mould while at the same time allowing air bubbles to escape. Pour the plaster slowly to prevent the air bubbles being there in the first place. I read somewhere that it's a good idea to pour some in and then rotate the mould around in all angles to coat the interior surface of the mould with plaster before pouring any more in. This was good advice and I wish I'd taken it; next time. After you've poured the plaster you can gently agitate/tap the mould to encourage the air bubbles to rise to the surface. I did this and you can see that it works quite well. Keep in mind the shape of the object you're casting though, air bubbles can get trapped in high points inside the cast, the idea is to get them outside.

Now comes the waiting. Put the whole thing aside and leave it alone until you're sure the plaster has set.

Don't touch it. Wait.

Keep waiting.

Are you sure the plaster should have set already?

When the plaster has set you can remove the Alginate gel from your creation. First you need to get the whole lot out of the container without damaging the cast. You can see that I just cut/tore the plastic container away and dropped it into the recycling bin. I damaged the Alginate gel slightly getting it out, but that doesn't matter I need to remove that too.

The Alginate is thick and rubbery, but can be cut easily. I used a pair of kitchen shears to trim away large chunks of it and then a utility knife to carefully trim it away from the plaster. I found that in some detailed sections such as in skin-folds the Alginate tended to stick but was easily enough removed with a sharp implement.

Argh! A finger is broken! How did that happen? It was a clean but not perfect break so it presumably occurred late in the setting of the plaster. I really don't know how it happened but I'm pretty sure it didn't happen while I was removing the Alginate. I do recall moving the mould at some point during setting because I'd put it in a silly place and maybe it broke then. Next time I'll follow the advice about not moving it.

No matter. Plaster glues readily and because it was a clean break I managed to repair it satisfactorily. You can see the air bubbles in the plaster. If you can imagine the image inverted you can see that they are all on the upward surface of the mould. If I'd made the plaster a little more runny, if I'd poured it more slowly, if I'd followed the advice about running it around the inside surface, and perhaps if I'd used a funnel to pour the plaster into each finger first this wouldn't have happened. On the upside you can see some of the detail of the hand in the plaster. All things considered I was pretty happy with it as a first attempt. I could have filled the holes if I'd been bothered but not this time, next time I'll take more care and do a better job.

Now to finish it off.

I used some acrylic paint to paint the hand. I'm no artist and frankly the acrylic paint was annoyingly thick so the paint job isn't fantastic, but I think the end-result is honest.

I've got a few ideas for things I'd like to use various casts for.

Next time I'll get the kids involved too, they'll love it.