When It Goes Wrong
Icing That Will Not Set Is Holding Water It Cannot Lose
Sugar icings set by evaporation or crystallisation, so humidity, excess liquid and a warm surface underneath each prevent them firming no matter how long they are left.
An icing that stays tacky for hours has not failed to set so much as been prevented from doing so. Something is keeping water in it.
Most icings set by losing water
A simple sugar icing is sugar dissolved beyond its usual limit, and it firms only as water leaves and the sugar returns toward solid form.
That requires the surrounding air to be able to accept the water. In a humid room the air already holds a great deal and takes very little more.
The same icing that sets in an hour on a dry day can stay wet all afternoon in damp weather with nothing else changed.
Excess liquid is the obvious cause
Sugar icings are extremely sensitive to their liquid, and a small addition produces a large change in consistency.
Once the ratio is too far toward liquid, evaporation has to remove a great deal of water before the icing reaches setting concentration.
The correction is more icing sugar rather than more time, added gradually since the same sensitivity applies in reverse.
A warm cake underneath prevents any set
Icing applied to a warm cake receives heat from below, which keeps the sugar mobile and can melt it into a thin transparent layer that soaks in.
The cake is also still releasing steam, which arrives directly at the underside of the icing and adds water to it continuously.
Waiting until the cake is fully at room temperature is not caution but a requirement, and it applies to every icing type.
Moisture from the cake keeps arriving
Even a cooled cake continues moving water outward, and a very moist cake will slowly soften the icing above it from below.
This is why glazed cakes are often finished close to serving, and why a crumb coat or a layer of jam is used as a barrier.
Fruit, syrup and anything alcoholic in the cake increases this considerably, since those components hold water loosely and release it steadily.
Some icings never fully set by design
Buttercream firms only as its fat cools and never dries, so it stays soft in a warm room and no amount of waiting hardens it.
Cream cheese icings behave similarly and hold more water besides, which is why they are kept refrigerated rather than left to firm at room temperature.
Expecting a fat-based icing to behave like a sugar-based one is the underlying error in most complaints about icing that will not set.
Questions readers ask
Does a cave mean the bread is underbaked?
No, since it is a structural fault rather than a doneness one, and the loaf is usually baked correctly. Check the internal temperature separately if you are unsure.
Can I still use a loaf with a big cave in it?
Yes, though the slices with the void in them are awkward for sandwiches. Toasting and cutting into smaller pieces makes it a non-issue.
