In the end some research turned up the following formula from the 1870s, which appears to work:
Milk Paint Formula
1 litre skimmed milk (at room temperature)
30 gm hydrated lime (by weight)
500 to 1200 gm of chalk may also be added as a filler
Stir in enough skimmed milk to hydrated lime to make a mixture with a creamy constituency. Add the balance of skimmed milk and stir well. Then add powder pigment(s) to desired colour and consistency (the pigment powder must be lime-proof). Stir in well for a few minutes before using. For best results continue to stir throughout use.
Apply milk paint with a cheap natural bristle brush. The wall/furniture needs to dry before applying the next coat. Excess paint may be kept for several days in the fridge until the milk sours. Any surface painted with milk paint should be allowed to dry thoroughly for 3-4 hours before use. For extra protection, a coat of linseed or stand oil can be applied to the finish once it has dried.
Note: the colour of the paint may change slightly depending on the colour/porousity of the object being painted, so test in an inconspicuous area first before applying to a large area.
Milk paint is extremely durable (much better than modern alkyd paints) and will resist many commercial paint strippers.
Scrit