Slade was most well known as a slate-writing medium. During his séances he would place a small slate with a piece of chalk under a table and would claim spirits would use it to write messages. According to Joe Nickell, Slade was repeatedly caught faking the spirit messages in his séances and he produced his phenomena by a variety of magic tricks.
Science writer Karen Stollznow has noted that:
"Slate writing was a simple parlor trick, often involving a double-sided chalkboard or a hidden slate upon which the "message" was already written. Many mediums were caught faking the practice, including Henry Slade, the man who discovered the phenomenon. Slade was writing these messages from the "dead" using tiny pieces of chalk held in the fingers of either hand, the toes of either foot, or his mouth."
In 1872, Slade was caught in fraud in New York by John W. Truesdell, who had two sittings with him. During the séance Truesdell observed Slade using his foot to move objects under the table, and writing on a slate. In a séance Stanley LeFevre Krebs employed a secret mirror and caught Slade swapping slates and hiding them in the back of his chair.
In a séance in 1876 in London Ray Lankester and Bryan Donkin caught Slade in fraud. Lankester snatched the slate before the "spirit" message was supposed to be written, and found the writing already there. He was prosecuted for fraud on October 1, 1876 in London and was sentenced to three months in prison. However, Slade made an appeal, which was sustained, on the ground that the words "by palmistry or otherwise" had been omitted in the indictment. Before he could be arrested on the new summons, he fled to America.
Johann Karl Friedrich Zöllner, Professor of Physics and Astronomy at the University of Leipzig conducted several controlled experiments, using Slade, to evaluate his claims of paranormal ability in 1877. Slade failed some of the tests carried out under controlled conditions but still succeeded in fooling Zöllner in several other attempts. Hereward Carrington in his book The Physical Phenomena of Spiritualism (1907) revealed the fraudulent methods (with diagrams of the rope tricks) that Slade used in the Zöllner experiments.
In 1882 in Belleville séance sitters caught Slade making "spirit" raps against the rung of his chair, using his foot to move a slate, writing "spirit" messages and substituting slates. He was also exposed as a fraud in 1885 by the Seybert Commission as it was discovered that the slates had prepared messages on them.
The magician Harry Houdini met the ex-medium Remigius Weiss in Philadelphia who had testified to the Seybert Commission that Slade's methods were fraudulent. According to Houdini he had given him the "best expose ever written of Slade's slate writings." Weiss also obtained a signed confession from Slade that all his spiritualist manifestations were deceptions performed through tricks and this confession was reproduced by Houdini in his book A Magician Among the Spirits (first published, 1924).
- Joe Nickell. (2007). Adventures in Paranormal Investigation. The University Press of Kentucky. p. 40. ISBN 978-0813124674
- .Karen Stollznow. (2014). Language Myths, Mysteries and Magic. Palgrave Macmillan. p. 107. ISBN 978-1-137-40484-8
- Carl Murchison. (1927). The Case For And Against Psychical Belief. Clark University. p. 242
- Gordon Stein. (1996). The Encyclopedia of the Paranormal. Prometheus Books. p. 705. ISBN 978-1573920216
- Joseph McCabe. (1920). Spiritualism: A Popular History from 1847. Dodd, Mead and Company. pp. 160-161
- Paul Kurtz. (1985). A Skeptic's Handbook of Parapsychology. Prometheus Books. p. 254. ISBN 978-0879753009
- Chung Ling Soo. (1898). Spirit Slate Writing and Kindred Phenomena. Munn & Company. pp. 105-106. "Dr. Henry Slade was, of course, identified and recognized as the principal slate-writing medium, but at various times he presented other phenomena, one of which was the playing of an accordion while held in one hand under the table. The accordion was taken by him from the table with his right hand, at the end containing the strap, the keys or notes at the other end being away from him. He thus held the accordion beneath the table, and his left hand was laid on top of the table, where it was always in plain view. Nevertheless, the accordion was heard to give forth melodious tunes, and at the conclusion was brought up on top of the table as held originally; the whole dodge consisting in turning the accordion end for end as it went under the table. The strap end being now downward, and held between the legs, the medium's hand grasped the keyboard end, and worked the bellows and keys, holding the accordion firmly with the legs and working the hand, not with an arm movement, but mostly by a simple wrist movement. Of course, at the conclusion, the hand grasped the accordion at the strap end, and brought it up in this condition. Sometimes an accordion is tied with strings and sealed so the bellows cannot be worked. This is for the dark séance. Even in this condition the accordion is played by inserting a tube in the air-hole or valve and by the medium's using his lungs as bellows."
- Kaku, Michio (1994). Hyperspace: A Scientific Odyssey through Parallel Universes, Time Warps, and the Tenth Dimension. New York: Oxford University Press. pp. 49–53. ISBN 9780195085143.
- Hereward Carrington. (1907). The Physical Phenomena of Spiritualism. Herbert B. Turner & Co. pp. 19-47
- Carl Murchison. (1927). The Case For And Against Psychical Belief. Clark University. pp. 242-243
- David Abbot. (1908). Behind the Scenes with the Mediums. The Open Court Publishing Company. p. 191
- Harry Houdini. (2011 edition). A Magician Among the Spirits. Cambridge University Press. pp. 94-100. ISBN 978-1108027489
- Andrew Neher. (2011). Paranormal and Transcendental Experience: A Psychological Examination. Dover Publications. p. 215. ISBN 978-0486261676
- Massimo Polidoro. (2001). Final Séance: The Strange Friendship Between Houdini and Conan Doyle. Prometheus Books. p. 189. ISBN 1-57392-896-8
- Blavatsky Foundation.org 6 page pdf by Walter A. Carrithers, Jr.
- Powerful Psychic Reading Spiritualists and Mediums
- Albion Michigan History Notebook Article on Henry Slade published in the Morning Star newspaper on October 17, 1993, pg. 6
- Internet Archive of Library Books "Spirit Slate Writing And Kindred Phenomena" by William E. Robinson Munn & Company (1898 copyright expired) This book was scanned into the public domain by Google.