“And in the CLXXXIXth Chapter he prays that he may not be obliged to drink filthy water or be defiled in any way by it. The rich man, even, was not certain that the appointed offerings of meat and drink could or would be made in his tomb in perpetuity: what then was the poor man to do to save his ka from the ignominy of eating filth and drinking dirty water?
To get out of this difficulty the model of an altar in stone was made, and models of cakes, vases of water, fruit, meat, etc., were placed upon it; in cases where this was not possible figures of the offerings were sculptured upon the stone itself; in others, where even the expense of an altar could not be borne by the relatives of the dead, an altar with offerings painted upon it was placed in the tomb, and as long as it existed through the prayers recited, the ka did not lack food.
Sometimes neither altar, nor model nor picture of an altar was placed in the tomb, and the prayer that sepulchral meals might be given to the deceased by the gods, which was inscribed upon some article of funeral furniture, was the only provision made for the wants of the ka; but every time any one who passed by the tomb recited that prayer, and coupled with it the name of the man who was buried in it, his ka was provided with a fresh supply of meat and drink offerings, for the models or pictures of them in the inscription straightway became veritable substances.”
E.A. Wallis Budge, Egyptian Magic, London, 1901. Pp. 106-7.