Evaporating, and why the last part is the hard part

Why the finish accelerates

As water leaves, there is less of it to carry away the heat and less volume to dilute each further loss. A batch that moved a fraction of a degree every few minutes early on will move through the whole legal window in the time it takes to answer the door. That is why the finish is done in a smaller pan, watched, with the instrument already in hand.

It only goes one way quickly

Overshooting is correctable by adding back water or thinner syrup, and every correction is another measurement and another chance to overshoot in the other direction. Undershooting is correctable by boiling longer. Neither is a disaster; both cost time and both are avoidable by approaching the number slowly rather than discovering it.

Scorching at the end

A concentrated syrup on a hot surface burns, and a burned note does not come back out. The risk is highest exactly when the batch is most valuable, which is the argument for a finishing pan with good heat distribution rather than the vessel you boiled in.

Temperature is a proxy, not the measurement

Syrup boils at a temperature above water that rises as it concentrates, so a thermometer tracks progress usefully. It is a proxy: the offset depends on the local boiling point, which depends on altitude and the weather that day. Calibrate against boiling water on the day if you use it, and finish on the density instrument rather than on the thermometer.

Decide the finish point before you start

Not at the pan, with a batch moving. Write the target down, aim at the middle of the window rather than the edge, and measure to confirm you are there rather than to find out where you are.

References