Why did my sponge not rise?

A sponge rises because air beaten into the fat expands, and because the raising agent produces gas. Anything that removes the air, kills the gas or lets the structure collapse before it sets gives you a flat cake, and the causes are distinguishable after the fact.

Step by step

1. Test the raising agent first.A teaspoon of baking powder into hot water should fizz immediately and vigorously. Bicarbonate needs an acid — use vinegar. A weak reaction means it is spent, and that is the commonest single cause.
2. Check how old it is.Baking powder loses strength within about six months of opening, faster in a damp kitchen. Self-raising flour has the same problem and is often much older than people realise.
3. Cream the butter and sugar properly.Genuinely pale and fluffy, which takes four or five minutes with a mixer and longer by hand. This step beats in the air the cake rises on. Butter at room temperature; fridge-cold butter will not hold air.
4. Stop mixing when the flour is just combined.Beating after the flour goes in develops gluten and knocks out the air. Fold gently until you can no longer see dry flour, then stop, even if it looks slightly uneven.
5. Check the oven's real temperature.Domestic ovens are commonly 20°C or more out. Too cool and the mixture spreads before it sets; too hot and it sets before it has risen. An oven thermometer costs very little and settles it.
6. Stop opening the door.The rush of cold air collapses a sponge that has not set. No opening for the first three-quarters of the baking time.
7. Get it into the oven immediately.The raising agent starts reacting the moment it meets liquid. A mixture left sitting on the counter for twenty minutes has used up much of its lift before it reaches the heat.
8. Check the tin size.A mixture for two 18cm tins spread across two 23cm tins gives a thin flat cake that was never going to look right. Match the tin to the recipe, or scale the mixture.
9. Use eggs at room temperature.Cold eggs added to creamed butter make the mixture curdle and lose the air already beaten in. Take them out half an hour ahead, or warm them briefly in water.
10. Weigh, do not estimate.Too much flour or too little raising agent both flatten a sponge, and both come from measuring by volume. Baking is the one place where the scales matter.
11. Rescue what you have.A flat sponge is still perfectly good cake. Layer it with cream and fruit, cut it into a traybake, cube it for a trifle, or turn it into cake truffles with a little buttercream.

Tips

• Write the opening date on baking powder and bicarbonate with a marker. It is the cheapest ingredient in the cupboard and the one most likely to be the problem.
• If the cake rose and then sank, that is a different fault — usually underbaking or the door being opened. If it never rose at all, work through the list above.
• Self-raising flour is plain flour plus baking powder, roughly two teaspoons per 150g. Making it fresh gives more reliable lift than an old bag.

Common mistakes

• Beating hard after adding the flour — It develops gluten and knocks out the air the creaming just built in. Fold gently and stop as soon as the flour disappears.
• Using butter straight from the fridge — Cold butter will not hold air, so creaming achieves nothing and the cake has no mechanical lift to rely on.

If it doesn't work

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