I was having a look to see what Mrs Grieve wrote in 1936 about Blackberries, she is usually a mine of information. She invariably has anecdotes from Gerard or Culpeper to support her opinions. In the case of the Blackberry, while bemoaning the multitude of hybrids that hinder identification, she mentions that the London Pharmacopoeia (1696) says that Blackberry Cordial “contains a notable restorative spirit.” She talks of Blackberry vinegar, made by covering the Blackberries in malt vinegar and leaving to stand for three days before straining, adding a pound of sugar to every pint of juice and boiling for five minutes, cool and bottle. A teaspoonful, mixed with water to be taken in cases of a feverish cold. She also mentions that Blackberry leaves, boiled with lye make a very effective black hair dye.
Lye is an assortment of chemicals, including caustic soda, that dissolve fat. This, as you can imagine, is quite dangerous to use. Naturally, people, being only human, use it to flavour and preserve various foods (including olives) and to clear blocked drains. To make lye you need a nice big heap of hardwood ash. You put it in a barrel (Note here – it will attack glass – so bottles and jars are out) and cover it with water. After a few days strain it off, now pour it back in and repeat as often as you like.
I had intended to give you a recipe for a nice Blackberry jam or jelly but I’m afraid making lye proved to be too tempting to ignore – so here’s one I found on a wee bit of cooking (amongst many other recipes) – will this do?