Concrete casting – first try

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I’d long wanted to have a go at making “stone” pots, troughs etc – hypertufa looked awfully complicated, with getting the right ingredients and mixing them properly in the right proportions, so I thought I’d get a bag of ready-mix concrete, just-add-water-and-stir, and see how I got on with that first.

Got a very small bag (it was the only one the hardware shop did that I’d be able to carry home!) and it sat there for months: got packed in the moved, unpacked, and sat there for more months.

I was trying to find a small mixer, but “small concrete mixer” turned up ones that held 160 litres, a tad much for craft casting, at least for a beginner small scale. Then I had an idea, and bought a large wide plastic jug and a twin-blade hand-turned whisk. That should do for mixing, I thought; if it does cake mixes, it should do concrete dust and water!

All sat there for another month or so, then one day I thought, Right! Do it today or get rid of the lot. So about 7am I got ready: bearing in mind the fineness of the powder and how close I’d have to get to see what I was doing, I took no risks: put a dust mask on, and wore long sleeves tucked into rubber gloves so my skin should be protected from splashes (there were warnings on the pack).

Didn’t really know what I was going to do with it; didn’t have a mould to cast into. I had some finely sieved dirt for exactly this purpose – I’d thought about putting it into a dish, pressing something into the dirt and putting the concrete in the “mould” – I could then smooth the dirt and press something else, so I wouldn’t need a rigid mould.

I picked a shell from one of the pots and pressed that in – the dish wasn’t quite deep enough but I decided to have a go anyway; tried pressing the shell in on different sides to get different impressions.

I couldn’t see any directions on the pack for what proportion mix-water to use, so I just chucked some water in the jug and added the mix carefully, bit at a time. The only thing I didn’t think of was how to scoop the powder so had to use a spare plant-food scoop – didn’t fancy using a kitchen spoon!

I wasn’t sure whether to add water to the mix, or add the mix to water; the pack instructions implied adding it to water, so put loads of scoops in, then gave the whisk a whizz. Then loads more, whizz, then loads more … the mixture didn’t seem to be thickening much. I had a tentative grope in the jug and found that the mix was ridging up around the bottom rim where the whisk wasn’t reaching. So grabbed a barbecue stick (all that was to hand) and stirred into the corners till it felt even.

The pack said to mix to a “smooth paste” but it didn’t look very paste-y; still very runny. I didn’t want it too thick to pour, so I decided to give this a go and see how it took.

Hit a problem immediately: black holes in black dirt are very hard to see! I managed to find them, more or less, and filled the space with all the concrete I’d mixed – there was a bit much, but not too much, all things considered.

Put them in the shed to set – the pack said it was very quick-drying (compared to what?). I checked them a couple of hours later and gave one a very gentle prod, it seemed firm already, but I’d read other people’s blogs about leaving concrete for a couple of days, so I left them all day and night; forgot about them the next day.

Took them out the third day. They don’t look much like shells, or even shell-shaped! But at least the method worked, more or less (rather less than more, I think). Maybe I should have used a less fancy shell, or at least a smaller one that would have fitted all the way down; maybe I should have firmed the dirt a bit more, or damped it a bit more – and certainly should have mixed a bit less concrete, or at least not have filled the holes so much that the excess overflowed.

But still, for a first go and working totally blind, I’m not hanging my head!

“model”

casting:

results:

There seems to be a lot of dirt embedded in the concrete: I gave them a cautious wash and a bit of a scrub, but that doesn’t seem to have improved them much. Maybe I could have added some colour to the water before mixing – it would have been rather hard to grease a hole in dirt to ensure the concrete didn’t stick!

The final results look more like stranded whales than shells, because of the overflow of extra mixture – the how-to sites say to have something in reserve to use up any excess mixture, but I didn’t think I’d mixed that much.

This was about half a pack – maybe I could have mixed it thinner, it set very fast in this concentration.

If anyone knows what the proportions should be, please share!
and any other tips?

• 14 Jun, 2014
• 4 likes

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