"one thing may be asserted of this work, which can be said of few others that have passed the hands of an engraver, which is, that Mr. Blake has condescended to take upon him the laborious office of making them, I may say fac-similes of my originals: a compliment, from a man of his extraordinary genius and abilities, the highest, I believe, I shall ever receive: - and I am indebted to his generous partiality for the instruction which encouraged me to execute a great part of the plates myself; enabling me thereby to reduce considerably the price of the book."
British Museum Illustration to Thoughts on Outline Cupid and Psyche
The value which Cumberland placed on Blake's work is indicated by the fact that he owned copies of seven of Blake's illuminated books: Book of Thel, Gates of Paradise, America, Europe, Song of Los, Visions of the Daughters of Albion, and Songs of Innocence & Experience. (Wiki)
Letters, (E 699) "G Cumberland Esqr, Bishopsgate near Egham, Surrey Lambeth, 6 Decembr 1795 Dear Sir I congratulate you not on any atchievement. because I know. that the Genius that produces. these Designs can execute them in any manner. notwithstanding the pretended Philosophy which teaches that Execution is the power of One & Invention of Another--Locke says it i[s the] same faculty that Invents Judges, & I say he who can Invent can Execute. As to laying on the Wax it is as follows Take a cake of Virgins wax <([if it can be found] [if such be]< I dont know what animal produces it>)> & stroke it regularly over the surface of a warm Plate. (the Plate must be warm enough to melt the Wax as it passes over) then immediately draw a feather over it & you will get all even surface which when cold will recieve any impression minutely Note The danger is in not covering the Plate All over Now You will I hope shew all the family of Antique Borers, that Peace & Plenty & Domestic Happiness is the Source of Sublime Art, & prove to the Abstract Philosophers--that Enjoyment & not Abstinence is the food of Intellect. Yours sincerely WILL BLAKE Health to Mr Cumberland & Family The pressure necessary to roll off the lines is the same as when you print, or not quite so great. I have not been able to send a proof of the bath tho I have done the corrections. my paper not being in order."
That Blake's final engraving was made at the request of George Cumberland
in 1827 is testimony to closeness they enjoyed. Other posts on Cumberland: Last Supper Reading Blake This Foolish Body