Royal icing
Icing sugar and egg white that sets by drying rather than by cooling or cooking — which is why humidity, not technique, is usually what went wrong.
What makes it set
Sugar drying out
A baked product is set by the oven. This one is not — and what sets it instead decides almost everything that can go wrong with it. See the whole family by mechanism.
Also called royal frosting (US), flood icing (INTERNATIONAL) — Royal icing thinned for filling an outlined area., line icing (INTERNATIONAL)
Royal icing is icing sugar beaten with egg white — fresh, pasteurised, or reconstituted from dried albumen or meringue powder — until it holds a peak. It is the icing that dries hard: piped lines that keep their edge, a surface firm enough to stack, decorations that survive being posted.
It sets by losing water. There is no cooking, no cooling, no crystallisation to manage; the water in the egg white evaporates, and what is left behind is a rigid mass of sugar with a little protein through it. Everything difficult about royal icing follows from that one fact. It sets slowly, it sets faster in dry air and barely at all in damp air, it forms a skin on the bowl within minutes, and it can be softened again by the weather months later.
Bakers who find it unpredictable are usually treating it as a mixing problem. It is a drying problem, and the room is part of the recipe.
What holds it together
A continuous, brittle sugar matrix formed as water leaves a saturated icing-sugar-and-albumen paste, with egg white protein contributing adhesion and some flexibility but not the set itself.
A statement of what the preparation is rather than a measured property. That the set is a drying process rather than a coagulation is evident from the fact that it proceeds at room temperature and reverses in humid air.
RecordedTrue by definition rather than by measurement
What temperature does to it
Much less temperature-sensitive than any fat-based icing and much more humidity-sensitive. It will not soften in a warm room the way buttercream does; it will refuse to dry, or dry and then go tacky again, in a damp one. Warmth helps only in so far as warm air holds and removes more moisture. A refrigerator is the worst place for finished royal icing, because taking it out puts moist air onto a cold sugar surface.
Follows from the setting mechanism recorded above and from sugar's hygroscopicity, which BakeHQ records on the sugar ingredient pages.
RecordedFollows from evidence recorded elsewhere on BakeHQ
How it keeps
Unused icing keeps for a few days covered airtight with the surface pressed down, because a skin forms within minutes of exposure and the dried fragments will not redissolve. Finished, dried decorations keep for months in a dry, sealed container away from anything moist — and will soften if stored with a cake.
RecordedEstablished practice, not a measured result
What a good one is like
- Structure
- Holds a piped line without spreading; dries hard enough to stack
- Texture
- Smooth and matt when dry, with no visible sugar grain
- Colour
- Even, with no darker patches or bleeding at colour boundaries
Where it stops
Royal icing is brittle by design and it will crack on anything that flexes or shrinks under it — a cake that is still settling, a biscuit that softens as it absorbs moisture from a filling, a surface that moves in transit. It cannot be made pliable without adding glycerine, and glycerine works by holding water in, which means it also stops the icing setting fully hard: the two properties are the same property. It is also not a covering for anything that needs to stay soft, and it is a poor choice in a humid kitchen at any skill level.
Bleeding, and the only thing that prevents it
Colours bleeding into one another is the failure that upsets people most, because the work looks finished and then quietly ruins itself over the following hour.
It is migration through a wet layer. While the icing still holds water, dissolved colour can move through it, and it moves from where there is more of it to where there is less — so a dark outline creeps into a pale flood, and a red bleeds into white far more visibly than the reverse. Nothing in the mixing prevents this. What prevents it is time: each colour has to have lost enough water to stop being a solution before the next one touches it.
Two things make it much worse. A damp room, because nothing is drying. And extra liquid colouring, because the colour was carried in water and that water has to leave too. Concentrated gel or powder colours bleed less for that reason rather than because they are better pigments.
Why it dries dull, blotchy or spotted
A royal icing that dries with dull patches, or with a faint mottling that was not there when it was wet, has usually dried too slowly.
The surface finish depends on the sugar coming out of solution evenly as the water goes. Dried quickly and evenly — moving air, dry room — it sets with a smooth, slightly satin face. Dried slowly, the water leaves unevenly, and the sugar sets in patches of different crystal size that scatter light differently. That is what the dullness is.
This is also why a fan or a dehumidifier is standard equipment for people who do a lot of this, and why the same recipe behaves differently in February and in August.
Related
Things that go wrong
Royal icing will not set, cracks or bleeds
Setting by evaporation is what all three failures are about, and royal icing is the only thing here that sets that way alone.
Used on and in
The decorating icing for a biscuit that has to stay stackable and postable, which is exactly what drying hard buys.
What it is built from
Nearly all of the finished mass. The set is this sugar left behind as the water goes.
Supplies the water that has to evaporate and the protein that makes the dried icing adhere rather than dust off.
Commonly confused with
Both are sugar coverings and they set by different routes — one crystallises or stays plastic, the other dries rigid — so advice written for either is wrong about the other.
Keep reading
Buttercream
Sets by solid fat rather than by sugar drying out, which is why the same complaint means something different in each.
Royal icing will not set, cracks or bleeds
Work through what you can actually observe, rather than a list of possibilities.
Fillings, icings and creams
The whole family, grouped by what sets each one.