It seems that most commonly in this era, the hair was washed with a gentle vegetable soap, no oftener than once a week and sometimes as seldom as once per month.
“To cleanse the hair, there is nothing better than soap and water . . . the soap, of course, should be mild, and well and plentifully rubbed in, and afterwards thoroughly removed with an abundance of water”-- Hints on Health, 1852
“Many heads of hair require nothing more in the way of wash than soap and water” -- Decorum 1879
“Supple moist oily hair may be washed every eight days with lukewarm water. Light hair, which is seldom oily, and the fineness and softness of which obviates the use of pomades, rarely requires washing. But a little honey dissolved in a very small quantity of spirit, scented with rosemary, &c. is an excellent substitute.”-- Mrs. Walker, 1840