Knowing the endpoint without a thermometer

Syrup makers working without instruments judge the endpoint by hand: a thread drawn between the fingers, a drop set in cold water, a flake falling from the skimmer, or an egg floated in the pan. This paper calibrates each of those tests against final solids concentration, boiling temperature at three altitudes, and final moisture content, using stated correlations for density and boiling-point rise, and it prices the error of each test in fuel and in product loss. The target used throughout is syrup at 70% solids and 30% moisture. The thread test, the cold-water test, and the sheet test each centre between 70% and 72% solids with an error of ±3.0 percentage points of moisture. A litre measure weighed on a kitchen scale does better, at ±1.0 percentage points. The floating-egg test, judged by a coin-sized area of shell exposed, does not survive as a precision test: its nominal centre is 72.5% solids, but combining the stated ranges for egg density and for the exposed area an observer calls a coin gives a range from 54.0% to 95.0% solids. From juice at 15% solids, finishing 1 kg of syrup at the 70% target costs 1.980 kg of wood on the stated open-pan assumptions. Stopping at 65% solids saves 8.8% of that fuel but leaves 50 g of excess water in every kilogram. Running to 78% solids costs 14.1% extra fuel and, for a pure sucrose syrup cooled to 20 °C, puts an upper bound of 331.3 g per kilogram into crystals that fall out of solution. The recommended household method is a weighed litre as the primary test, with the thread test as a cross-check. All apparatus figures are SIMULATED, NOT MEASURED.