Occasionally, during the demoulding of a form, the mould will break. Not all is lost. It can be repaired.
In this example, the mould is not yet fully cured and is damp. But this can be applied to fully cured and dried moulds too. Notes will be included where the practice varies for the dried mould.
The first stage is to make up a paste of the ciment fondue for the edge to edge repair. This should be the consistency of pancake batter or slightly wetter. The mixed cement is shown at the top of the picture in a small plastic tub.
Wet the edges of the mould pieces thoroughly. This is to prevent the mould from sucking too much water from the cement, which would give a weak adhesion. On dried moulds, you may have to do this several times to thoroughly wet the mould and the broken piece.
Then begin applying the wet cement thinly to all the edges. Do not put it on thickly, as you want the pieces to fit back together smoothly.
Place the pieces together with gentle pressure.
Then begin to smooth the wet ciment fondue into the cracks between the broken pieces and the main body. Be careful to smooth the ciment fondu immediately, as it is very difficult to change once cured.
Continue to work the ciment fondue into any cracks that appear as the mould is wetted.
Make sure the cement is smoothed into the cracks so there are no proud areas above or around the cracks.
This photo shows the smoothed ciment fondu on the interior.
Continue smoothing the cement into the cracks at the edges.
Fill the cracks from the outside also
When the application of the cement is completed, make up a mixture of 1:4 ciment fondue to vermiculite.
The purpose of this is to strengthen the mould in the weak area. It is not wise to rely entirely on the strength of the edge bonding of the ciment fondue.
You will need to estimate the total volume required, but it is better to mix too much rather than too little. Make this mix a little wetter than for the original mould. Water should not be standing in the mix, but you will be able to squeeze water from the ball of mix easily.
This is especially important for moulds which have already been cured. You should also put water on the surface that you are going to back up.
It is important to put a water proof material on the workbench to avoid the mould sticking to the bench, or water dripping over other things.
Having wetted the mould exterior again, begin applying the mix to the outside of the mould.
Continue building up the mixture in thin layers. This allows the best adhesion of the material to the mould and to each layer. It is easier to compact a small amount of material than a large amount all at one time.
In this photo, you see some of the water being forced out of the mixture by the compaction of the mix onto the mould.
Continue building around the broken area until you have applied sufficient material to the mould to strengthen it.
When you have finished, one area of the mould may be a little larger than the rest. This is not a problem in its use, as it does not thermal shock, and it does not keep one part of the glass hotter than the glass touching the rest of the mould.
You can now loosely wrap the water proof material around the mould. Do not seal it completely. Place the mould in a plastic bag to cure for a day or more, just as for the original mould.
You can then unwrap the mould and fire it to cure it just as the original. The method for curing vermiculite moulds is given here.