A great many sweets depend on sugar syrup cooked to a particular concentration. Traditional instructions describe this through physical tests, and understanding what those tests measure makes the whole thing considerably less mysterious.
What's being measured
As sugar syrup boils, water evaporates and sugar concentration rises. Concentration determines everything about the final texture.
Boiling point rises with concentration — a solution boils above 100 degrees, and the more concentrated it is, the higher the boiling point.
Which means temperature is a direct proxy for concentration. Every traditional stage description corresponds to a temperature range.
This is why a thermometer works and is genuinely more reliable than the traditional tests for anyone who hasn't done it hundreds of times.
The stages
Approximate temperatures at sea level, and adjust for altitude — boiling point drops with altitude, so target temperatures should be reduced accordingly.
Thread stages, roughly 106 to 115 degrees. Syrup drawn between finger and thumb forms threads. One thread at the lower end, two at the higher. Used for soaking sweets, for jalebi syrup, for many preparations where the syrup remains liquid.
The traditional test involves touching hot syrup, which requires care. Cooling a drop on a cold plate first is safer and works.
Soft ball, around 112 to 116. A drop into cold water forms a soft ball that flattens when removed. Used for fudge-like textures and several barfi preparations.
Firm and hard ball, roughly 118 to 130. Progressively firmer balls in cold water. Used for chewier confections.
Soft crack, around 132 to 143. Threads that bend before breaking.
Hard crack, around 146 to 154. Threads that snap. Used for brittles and hard sweets.
Caramel, above 160. Sugar begins to break down and colour, producing caramel flavour. Beyond this it burns rapidly.
Why the margins are narrow
Two reasons this is difficult.
The temperature rises slowly at first, while there's plenty of water, and then accelerates sharply as the syrup concentrates. The last few degrees can happen in seconds.
And residual heat continues cooking after removal from the flame. A syrup taken off at a target temperature will continue rising for several degrees, which means you generally remove it slightly before the target.
These together mean that inattention at the end of the process ruins a batch that was fine two minutes earlier.
Crystallisation and how to prevent it
The most common failure. Concentrated sugar syrup is metastable and readily crystallises, producing a grainy texture where smoothness was intended.
Triggers include undissolved sugar crystals, crystals on the pan sides falling back in, agitation, and rough surfaces.
Preventions:
Dissolve completely before boiling. Stir over low heat until no grains remain, then stop stirring.
Don't stir after boiling starts. Agitation encourages crystal formation.
Wash down the pan sides. A wet pastry brush around the interior above the syrup line dissolves crystals that would otherwise seed the batch.
Add an interfering agent. A small amount of acid — lemon juice or citric acid — partially inverts the sugar, producing glucose and fructose which interfere with sucrose crystal formation. Glucose syrup does the same directly.
Traditional recipes calling for a few drops of lemon in the syrup are doing exactly this, whether or not the reason was understood.
Deliberate crystallisation
Some preparations want a fine crystalline texture rather than a smooth one.
Achieved by cooking to the appropriate stage and then agitating vigorously as it cools, which produces many small crystals rather than few large ones. The vigour and timing of the beating determine the grain size.
Getting this wrong in either direction produces either a smooth result where grain was wanted or coarse grains where fine ones were.
Practical equipment
A digital thermometer with a probe is inexpensive and removes most of the difficulty.
A heavy pan distributes heat evenly and prevents hot spots that scorch.
A pan larger than seems necessary, because sugar syrup foams up dramatically at certain stages and boiling over is both dangerous and messy.
And a bowl of cold water beside the pan, both for testing and because sugar burns are serious. Molten sugar sticks to skin and continues transferring heat, which is why it produces worse burns than boiling water at a similar temperature.
Anyone working with syrup above the thread stages should treat it with genuine caution, keep children away from the pan, and have cold water immediately available.
Rescuing a batch
Some failures are recoverable and knowing which saves a great deal of frustration.
Syrup taken off too early can simply be returned to the heat and cooked further. No harm done.
Syrup that has crystallised can frequently be rescued by adding water, returning to the heat, and dissolving everything completely before starting again. You lose time and not the ingredients.
Syrup that has gone past the target stage cannot be brought back — you cannot remove concentration. Adding water and recooking is possible in principle and the sugar will have started to caramelise, which changes the flavour permanently.
And syrup that has burnt is finished. Burnt sugar is intensely bitter and no amount of dilution removes it. Start again, and wash the pan properly, because residue will affect the next batch.