The Lake Ice Harvest: Ice Growth by Date, Tonnes per Hectare, and the Labour in a Tonne of Hand-Cut Ice

Before mechanical refrigeration, winter ice cut from lakes and stored under sawdust was the cold supply for dairies, breweries, fish stores, and hospitals. Whether such a harvest is worth organising in a place without reliable electricity depends on three practical questions: when does the sheet become thick enough to cut, how much ice does a hectare of lake actually yield, and how much human work does each delivered tonne contain? This study answers all three with a single transparent model. Ice growth is computed with the Stefan freezing equation, driven by a stated sequence of winter temperatures totalling 1615 freezing degree-days between 20 November and 1 April, for a bare sheet and for a sheet carrying a constant 0.15 m layer of snow with thermal conductivity 0.25 W/(m K). The bare sheet reaches the stated harvest thickness of 0.30 m on 2 December and 1.416 m by 1 April. Under snow the same dates give far less ice: 0.30 m is not reached until 16 January, and the final thickness is 0.616 m, a reduction of 56.5 percent. At 0.30 m thickness one hectare holds 2751.0 tonnes gross and 2475.9 tonnes after a stated 10 percent allowance for kerf, breakage, and trim. The labour account is dominated by sawing. Blocks of 0.55 m by 0.55 m require 13.22 m of saw cut per tonne, which at a stated hand-sawing rate of 4.0 m per person-hour takes 3.30 person-hours per tonne; freeing, loading, hauling, and stacking add 0.83, 1.11, and 0.67 person-hours, for a total of 5.92 person-hours per tonne delivered to the store. A twenty-person crew working ten-hour days therefore needs 73.2 crew-days to clear one net hectare, and the snow-covered case shortens the harvest window from 109 days to 64 days. The conclusion is plain: in the stated climate the ice itself is abundant, and hand labour, not cold, is the limiting resource. Snow left on the lake is expensive twice over, because it delays the start by six weeks and shrinks the season in which the work can be done.