One recipe does not fit every fat

Household soap recipes are often transmitted as a single quantity of alkali per unit of fat, for use with whatever fat is at hand. This paper tests that practice against published saponification values for six common fats and shows, in numbers, that it does not survive. A fixed recipe of 140.0 g of sodium hydroxide per kilogram requires, in fact, 99.1 g for lard and 182.5 g for coconut oil, a range whose maximum is 1.84 times its minimum. On tallow the fixed recipe leaves 39.5 g of surplus alkali per kilogram, a free alkali content of 3.06% in the cured bar, 30.6 times a 0.1% reference limit. On coconut oil it falls short by 42.5 g and leaves 23.3% of the fat unsaponified. No single fixed quantity satisfies both extremes: the ceiling from the lowest-demand fat is 100.4 g per kilogram, while the floor from the highest-demand fat, allowing at most 8% unsaponified fat, is 167.9 g per kilogram, a gap of 67.5 g. The replacement rule is fat-specific: dose 95% of the requirement calculated from the fat's published value, giving 95.5 g per kilogram for tallow, 173.4 g for coconut oil and 118.8 g for a stated 70/30 tallow and coconut blend.