While on holidays in Portugal, we bought a set of azulejos tiles, picturing an ancient windmill. The idea was to use them as a wall decoration in the garden. Just like you see at many places in Portugal. At home i had to thinker about how to do this. Fitting the tiles directly to the garden wall seemed not a good plan. So i decided to make a plaque for the tiles that i can hang against the wall, or put it on the ground somewhere.
The picture on the right shows a simulation that i made with 3d-builder. This helps to get an idea of the final result.
Since this plaque should be weather and winterproof there is no other choice than to make it out of concrete. Concrete however is a very heavy material. Since this plaque has to be fairly strong, so also fairly thick, it would mean that it would become a very heavy object. This isn't very handy since the idea is to hang it up to the wall. After some research on the internet i discovered a lightweight concrete that might be just suitable for this project; Hypertufa. This concrete has the looks of natural rock and gets a nice patina when leaving it outdoor. Moreover it would be winter proof so that we can leave it carefree outside in winter.
For the concrete we need:
For the mold
The plaque has to be as strong and winterproof as possible. These characteristics can be influenced by the shape, the concrete mixture and the mold.
As for the shape, this can't be too thin as this will make it vulnerable. We don't want it to break with our valuable tiles on it. Its worth the effort to think about some kind of inforcement for the concrete.
So the first thing we need is good mold. This mold should be rigid and inert to moisture so that the cast will not be exposed to crimp or bend effects in the mold material. Time to sit behind the drawing table. I used some 3d drawing software to design the rough idea of the plaque.
Once i was satisfied with the rough shape, i had to think about how to decorate the fairly dull shape. After all it should not look too much like a grave stone. The decorations can be made of a removeable material (silicone, albast cast, plaster) and can be fitted in the mold. Like a sort of stamp they will leave their print in the cast. This is the easiest way without making the mold too complex.
Now how can we make the mold? My initial plan is to use a firm piece of mdf and fix the mold on that. I could make the edges of a flexible material and make this rigid with the help of wall plaster that can be easely removed afterwards. It is possible to first make a model of plaster or cley and use that to make the mold. But that involves a lot of extra work.
I started with a board of mdf, roughly sawed into the desired dimensions. The tiles are not symetric so in order to mark the place where they will come, i layed the tiles in the right position on the board.
Now i drawed the shape of the tiles on the board, taken the joint seams into account. From this drawing off i measured out the outside of the plaque.
The mold is only once needed so i decided to make it out of plaster. This can be destroyed afterwards so getting the plaque out of the mold shouldn't be a problem.
In order to give the mdf some protection against the moist in the plaster and concrete, i painted the plates with thin epoxy resin.
Now i temporarely mounted some raised edges on the board, to which i am going to apply the plaster. The planes where the plaster will be applied are finished with packing tape, the plaster will not stick to that. For the bow i need a flexible material, a plastic strip or somethinglike that.
In the mean time i experimented with hypertufa. This would give a lightweighted result.
The used mix of the materials is:
1 part portland cement, 2 parts peat moss and 3 parts perlite. I mixed the dry material and applied water bit by bit to get the desired thickness. Then i modelled the cley-like mud into plastic houshold containers. The result however gives a very rough surface. I finished one of the pots with a dry rub of portland cement. Maybe this is a way to obtain a smooth surface. We'l see..
I have doubts about the strength of this material but casting the whole thing out of concrete would make it too heavy.
Now the mold is ready i can calculate the content, this would be some 9 + 5 = 15 liter. Out of concrete it would weigh 36kg. This is too much! So what i have in mind is to make the kernel out of hypertufa and the parts that come in sight out of concrete. And inforce it with a raster of steel wire. This should be doable by modelling the kernel out of the clay-like hypertufa and than surround it with a cast of concrete.