That they used practically the same system which has been adopted by all their followers is amply proved by the fact that after trial by ordeal had been abolished Albertus Magnus, in his work De Mirabilibus Mundi, at the end of his book De Secretis Mulierum, Amstelod, 1702, made public the underlying principles of heat-resistance; namely, the use of certain compounds which render the exposed parts to a more or less extent impervious to heat.

Many different formulas have been discovered which accomplish the purpose, but the principle remains unchanged.

The formula set down by Albertus Magnus was probably the first ever made public: the following translation of it is from the London Mirror: Take juice of marshmallow, and white of egg, flea-bane seeds, and lime; powder them and mix juice of radish with the white of egg; mix all thoroughly and with this composition annoint your body or hand and allow it to dry and afterwards annoint it again, and after this you may boldly take up hot iron without hurt.
"Such a paste," says the correspondent to the Mirror, "would indeed be very visible." Another early formula is given in the 1763 edition of Hocus Pocus.
Examination of the different editions of this book in my library discloses the fact that there are no fire formulas in the second edition, 1635, which is the earliest I have (first editions are very rare and there is only one record of a sale of that edition at auction).

From the fact that this formula was published during the time that Powell was appearing in England I gather that that circumstance may account for its addition to the book.

It does not appear in the German or Dutch editions.
The following is an exact copy: HOW TO WALK ON A HOT IRON BAR WITHOUT ANY DANGER OF SCALDING OR BURNING.
Take half an ounce of samphire, dissolve it in two ounces of aquaevitae, add to it one ounce of quicksilver, one ounce of liquid storax, which is the droppings of Myrrh and hinders the camphire from firing; take also two ounces of hematitus, a red stone to be had at the druggist's, and when you buy it let them beat it to powder in their great mortar, for it is so very hard that it cannot be done in a small one; put this to the afore-mentioned composition, and when you intend to walk on the bar you must annoint your feet well therewith, and you may walk over without danger: by this you may wash your hands in boiling lead.
This was the secret modus operandi made use of by Richardson, the first notably successful fire artist to appear in Europe, and it was disclosed by his servant.[1] Hone's Table Book, London, 1827, page 315, gives Richardson's method as follows: It consisted only in rubbing the hands and thoroughly washing the mouth, lips, tongue, teeth and other parts which were to touch the fire, with pure spirits of sulphur.