Nearly every casting fault traces back to one of four things: the ratio, the mixing, the pour, or demoulding too early.
Work through the question that matches what you are holding. None of these mean the material failed — they are the normal ways a cast goes wrong, and each one is a variable you can change.
Why does my cast have air bubbles?
Small pinholes across the face
Air folded in during mixing, or carried in by the pour. Mix with the spatula below the surface, pour a thin stream from low down into the lowest point of the mould, and tap the filled mould firmly to bring air up.
Shallow pinholes can be sanded back or filled with a little fresh mix and sanded flat.
Bubbles concentrated in the detail
Air trapped against fine relief that the mix could not displace. For textured moulds, work a small amount of mix into the detail with a soft brush before filling the rest.
Large voids inside the piece
Usually a mix too stiff to flow into the whole mould, or a pour that folded over itself. Check the ratio, and pour more slowly into a single point.
These cannot be repaired invisibly. Break the piece up for terrazzo chips and cast it again.
Why did my cast crack?
It cracked in the mould, before demoulding
Most often uneven drying — one part of the piece losing moisture faster than the rest, usually from heat, a draught or direct sun on one side. Set pieces somewhere stable, level and out of direct heat while they cure.
It cracked as I took it out
Demoulding too early, or levering the piece rather than releasing the edges. Peel the silicone back all the way round first, then invert and let the piece drop out under its own weight.
It cracked days later
Look at the form rather than the pour. Sharp internal corners and abrupt changes from thick to thin sections concentrate stress. Designing for moulds covers how to avoid that.
Why is my cast still soft?
It has not hardened in the expected time
The most likely cause is too much water — either a mis-measured ratio, or water left in the mould before pouring. Weigh both parts in grams rather than estimating by volume.
Cold and damp conditions can also slow things down. Most pieces can be demoulded after 25 to 30 minutes, but thick pieces and cool rooms take longer, so give it more time before deciding it has failed.
The surface is firm but the centre is soft
A thick section takes longer than a thin one, and the outside always gets there first. Wait. Demoulding on the strength of the surface is what breaks thick pieces.
Why are the edges chipping?
The rim broke away during demoulding
The rim is the thinnest, most exposed part of most pieces and it gains strength last. Release the whole rim from the silicone before lifting anything, and wait longer than feels necessary.
Edges chip while sanding
Too much pressure on an edge, or starting with paper that is too coarse. Work along an edge rather than into it, and ease sharp corners slightly instead of trying to keep them perfectly crisp.
Why is my colour different from what I mixed?
It dried lighter than it looked wet
Expected. Mineral materials generally lighten as they lose moisture, so wet mix is never a reliable preview. Judge colour on a fully dry test tile in daylight.
The colour is uneven or streaked
Incomplete dispersion. Add pigment to the measured water and mix it through before the powder goes in, and keep mixing until the colour is genuinely even.
Two pieces of the same colour do not match
Something changed between them — pigment quantity, ratio, mixing time or drying conditions. This is why written notes matter more than good judgement.
When can I sand it?
How long should I wait?
Long enough that the piece is hard and dry throughout. A piece still holding moisture clogs abrasive paper, finishes unevenly and is easier to damage at the edges.
Wait for the full cure, 24 to 48 hours, before sanding. Thick pieces and cool rooms need the longer end. If in doubt give it longer — nothing is lost by waiting.
