Royal icing is one recipe and three consistencies. Almost every decorating frustration comes from using the wrong one for the job, and almost every recipe describes them with words like "toothpaste" that mean something different to everyone.
There is a better test, and it takes ten seconds.
The second-count test
Drag a knife through the surface of the icing and count how long the line takes to disappear.
| Consistency | Line closes in | Use it for |
|---|---|---|
| Stiff | Never | Flowers, sturdy detail, holding things upright |
| Piping | 15–20 seconds | Outlines, lettering, fine lines |
| Flood | 8–10 seconds | Filling an outlined area |
Getting there
Start stiff and let it down. Adding water is easy; taking it back out means more icing sugar, and you end up with three times as much icing as you wanted.
- Make the icing stiff.
- Take out what you need for detail work and cover it immediately - royal icing crusts fast.
- Add water to the rest a quarter teaspoon at a time, stirring gently, until you hit the count you want.
Stir rather than beat. Beating whips air in, and air becomes the pinholes that appear as a flooded biscuit dries.
Two problems worth naming
Craters
A flooded surface that sinks in the middle usually means the icing was too thin, or it went on too thick. Both hold water that has nowhere to go. A drying fan or a dehumidifier helps far more than extra time does.
Colour bleed
Dark colours creeping into pale ones means the base layer was not dry when the next went on. Wet-on-wet is a technique in its own right, but if you want crisp separation, leave the base at least six hours - overnight if the kitchen is at all humid.
Gel colours, not liquid. Liquid food colouring thins your icing while you are colouring it, which quietly undoes the consistency you just measured.
On embossed biscuits
If your biscuit already carries an embossed pattern, resist flooding it flat. A thin brush of edible lustre, or piping only the recessed lines, keeps the detail you baked in. Flooding an embossed biscuit fills exactly the grooves that made it interesting.