Sometimes an alchemist practised both, using the profits of his sleight-of-hand to procure the means of carrying on his serious alchemical researches.

The impostures of some of these jugglers deceived even the most intelligent and learned men of the time, and so kept the flame of hope constantly burning.

The age of cold investigation had not arrived, and it is easy to understand how an unscrupulous mediaeval Hermann or Kellar might completely deceive even the most intelligent and thoughtful scholars.
In scoffing at the credulity of such an age, it should not be forgotten that the "Keely motor" was a late nineteenth-century illusion.
But long before the belief in the philosopher's stone had died out, the methods of the legerdemain alchemist had been investigated and reported upon officially by bodies of men appointed to make such investigations, although it took several generations completely to overthrow a superstition that had been handed down through several thousand years.
In April of 1772 Monsieur Geoffroy made a report to the Royal Academy of Sciences, at Paris, on the alchemic cheats principally of the sixteenth and seventeenth centuries.

In this report he explains many of the seemingly marvellous feats of the unscrupulous alchemists.

A very common form of deception was the use of a double-bottomed crucible.