There’s videos I planned to make which I’ve literally left in the corner of the workshop for quite some time. This is one such example. The idea was to make a picture frame out of concrete and this was the mould for it.
I’m going to put a bit of this chopping board oil in to try and reduce the likelihood that it will stick.
You can also use Vaseline as a release agent, or make the mould out of melamine backed sheet material or even lines the inside with parcel tape. These were all good suggesting from my followers on Instagram.
The nice thing about using a mould made out of real wood, is that you get the texture of grain on the final mould. You may have noticed this effect on some old brutalist buildings which have used wooden shuttering to contain the pours. It’s very Scandinavian and very attractive.
I’m going to use premixed patching cement for this attempt. This mix will have a high amount of Portland cement which will make the concrete set very hard, but also very quickly. The mix is 3 powder to 1 water and you always pour water into the dry mix, and not the other way around. This helps prevent lumps in the mix. I use to have a trowel somewhere and I don’t know where it is. I’m wearing gloves because cement is notorious for f*%^&ing up your hands. Keep them nice and soft for you…tube. And if you want you can also where a dust mask as well, in fact I’m going to put a dust mask on now coz I can see.
The other thing to remember is cement will not set if it is being agitated unlike plaster which sets when agitated. So keep stirring until you start pouring.
I did my mould in several pours and also used nylon fishing wire as a reinforcement. I didn’t get the mould out in time and as it set, the concrete contracted and broke in all four corners. So I did what any sensible person would do and stuck it back together with wood glue.
Surprisingly that stuck quite well, but I got too many air bubbles under the lip of the frame. I’m going to use this one to test a polyurethane varnish finish on the cement, and remake the mould with a different mix.
This is a polyurethane water based varnish. I’m going to use this to varnish the concrete and get an idea of how it will look. There’s all sorts of sealers for concrete. You can even just mix waterproof PVA and water together.
For the second attempt I picked a different brand of premixed concrete – mainly because it was on offer but also it has a medium range setting time. It is a flexible floor leveller which means it contains some kind of plasticiser to prevent micro fractures but also it must get mixed to a runny consistency to act like a leveller. So this should get into all the corners.
It was the only one between the two that I could find. It was either an hour or 24 hour set time. Obviously an hour’s too quick and what happen before was the cooling process meant that it contracted very quickly and formed the cracks. I don’t want to leave it like a typical brick mortar for 2-3 days to set and anyway that stuff is generally quite powdery so hopefully this is a good inbetween mixture. 1000 ml of water per 5 kg. So for 4 kilos I need about 800 ml.
I used corner brackets this time, along with the nylon thread to reinforce the concrete. I also drilled a few holes into the top of the mould so any air under the lip could be released, but the mix was a lot more runnier so I may not have had to do that.
Anyway to cut a long story short I broke this one as well so lets move along to the third attempt.
I think I’ve made a piece of art work, and I’m not saying that sarcastically. I think I have.
Ok I’m going to have one last go at this. What I’m going to do is clean up the mould and reassemble the outside as I had before, like that. I’m going to change this a little bit. I’m going to cut away maybe in the centre, just so that I can dismantle this a little more easily.
I scrapped and hoovered, oiled and reassembled my mould. This time I used hot glue to hold the central pieces in position. I’ll be able to pull these apart without having to unscrew anything. I am going to take the mould apart before the concrete fully sets as well.
I should have cut these at a slight angle to make it easier to pull these out. But it’s pretty straight so I shouldn’t have a problem. If I’d done it the opposite way around I wouldn’t be able to pull it out.
Ok I’m going to wait about 10 to 20 minutes while this starts to thicken up. I’m going to pop the bits of metal in just to reinforce it and the corner brackets as well. This time I’m going to make sure these go a lot closer to the inside edge than the outside, because if a little bit chips out here you’re not going to see that on the inside.
These are starting to feel quite hard so I’m going to try and open up the mould. At least the central bit. I’ll leave the outside as it is.
I think I might leave this bit exposed so you can see into the frame while it’s up. Ok it looks pretty good. There’s an ever so slight hair line fracture there but it’s kinda held. And that hasn’t gone through to the other side. There was a few places where the cement had broken off, and that was more to do with how I was dismantling the mould. You can see a little bit there, and another section there but I was able to glue them back. This particular mix was a little powdery than the first one I did which is down here. You can also see a slight difference in the colour, despite that I varnished this one it’s a lot more greyer. The other two which had a slower set mix, which means there was less portland cement in there. They look a bit more sandy. It got a kinda greeny yellow colour to them, which is basically the builders or sharp sand. I think if I do this again in the future I’m going to make my own mix and what I’ll do is use sharp sand. Maybe 2 sharp sands for 1 portalnd cement, or or maybe even 1 sharp sand to 1 cement. I think I’d get a better colour as well. Now I could, if I has some cement I could just mix that up with a bit water and maybe brush that in see if that binds and hide some of the imperfections but I don’t, and I’m not planning to go down to the DIY shop for a little while, so I think what I’m going to do, like the first one is paint it with polyunsaturate varnish. I think it needs a few coats, and what I need to do is, sand it back band hand after it’s dried. Just because the brushing process has lifted a bit of grit and dust off the surface of the concrete, and mixed that with the varnish. I think the finish could look a little bit nicer. Still not too happy with the colour of it. It looks too wet, too green. I much prefer the colder look of the mix with more portland cement. Once you put int his case the plastic in, you can see the depth of the frame is a little too large. But there’s plenty of room to place a piece of work, a drawing or something and a backing board, and obviously you can’t tack it as you would normally do with a frame – you would have to probably use silicone sealant or some kind of glue. There other things I need to think about in terms of hanging, and finalising the mould so the ratio looks a bit better in terms of depth of the lip to the actual depth of the frame. But as a test of the principle whether it’s possible to make a rectangular frame with a rebate out of concrete – it seems it’s possible.