What this step adds; in part B, the lye and the soapParts already in placeA soap yard on one base. Wood ash in a barrel over a filter of gravel and straw is soaked with boiling water; the lye that drips out gets stronger the longer it stands, and lye that floats a fresh egg is soap strength (Practical Action). The model's soak is an assumption: strength climbs as 1.2 × (1 − e^(−h/8)) in egg strengths, so the egg floats after about 14 hours. The batch follows the old rule of about 2 lb of grease to a gallon of lye water (0.24 kg per litre): 20 L of lye to 5 kg of fat. The boil is also the model's: saponification climbs as 1 − e^(−t/1.5 h) and trace is called at 90%, after about 3.5 h. Ash lye is mostly potash, which gives a soft soap; a handful of salt stirred in at the end firms it for bars. 10 bars of 10 × 10 × 5 cm cure on the rack. The tippy tap's 5 L can gives about 20 washes at 250 mL each (the sourced range is 50–500 mL). A Cochrane review of 14 randomised trials found handwashing promotion cut diarrhoea by about 30%; in 1847 Semmelweis's chlorinated-lime handwashing cut childbed-fever deaths in his Vienna clinic from 18.3% to 1.2%. This is a guide to how the parts go together, not engineering drawings. Lye burns skin and can blind: wear goggles and gloves, keep vinegar and plenty of water at hand, keep children away, and never use aluminium pots, spoons or containers, which lye eats.