Green’s Dictionary of Slang

Holt’s mixture n.

[Gen. Joseph Holt, Irish rebel leader in 1798]

second-rate gunpowder.

(con. 1798) J. Holt Memoirs 50: ‘Holt's mixture’ would have been perfect gunpowder, if he could have granulated it. From the want of granulation a considerable portion must have been blown away unfired.
(con. 1798) F.C. Armstrong Cruize of the Daring 119: She [...] had a mode of altering her countenance so as to appear a decrepit old hag; she deceived the military, especially the militia, and used to carry about her person a quantity of gunpowder, called Holt’s Mixture.