Flynn had a theory, and he returned to Arizona to test it. If Hofmann was a forger, he must have aged his documents artificially. That would have required exposing the documents to certain chemicals. And Flynn guessed that the chemicals also cracked the ink. Exactly which techniques Hofmann would have used, Flynn couldn't guess.
Flynn was 40; he had pursued forgers for half his life. He loved the hunt. At times he has even felt affection for the best forgers because they made the hunt better. And never had there been anything like this; nothing so tricky, nothing where the stakes were so high. Flynn realized that if he and Throckmorton returned a finding of forgery, they would be implying that the other examiners had been played for fools. In the beginning, Flynn had hesitated to take the case, knowing the problems that might follow. But he hadn't hesitated long.
Back in Arizona, Flynn realized he had entered unexplored territory. Other experts knew very little about artificial aging. Finally he pulled a volume from his bookshelf that only a documents examiner would own and love: "Forty Centuries of Ink." Inside he found references to 19th-Century frauds and some clues about the art of making new documents appear old. It seemed that all techniques of artificial aging involved the rapid oxidation of ink, in this case the iron gallotannic ink widely used a century ago.
Flynn knew that many chemicals would oxidize the iron component in this ink, and he figured Hofmann would have used a product that was easily available. So he tried household ammonia; it worked. Then he tried sodium hydroxide; it worked even better. The fumes from sodium hydroxide turned the black ink to a dark, red, old-looking color within minutes. The iron in the ink was converting to rust, just as it would convert naturally over a period of decades. The aging that resulted from the chemicals was not so much artificial as it was vastly accelerated.
Then came the real test. Flynn slipped one of his newly aged sheets under the microscope to check for the characteristic cracking; he pulled the letters into focus. The surface he saw was completely smooth. The cracking wasn't there. Flynn waited, hoping the cracking would take time to develop. He looked again; it still wasn't there.
Flynn was unsettled; he had been so sure. He called Throckmorton in Salt Lake and the two men discussed what might have gone wrong. They decided the likeliest culprit was the formulation of ink. Maybe Hofmann's ink was different from theirs.
They needed a break, and they got it. Flynn recalled that a book had been seized at Hofmann's house during a search; it was "Great Forgers and Famous Fakes," by Charles Hamilton, a New York documents expert. Flynn got a copy in Phoenix and thumbed through the pages. In the back of the volume was the page he had been hoping to find: a formula for 19th-Century iron gallotannic ink. The formula contained several ingredients that Flynn had not used in his composition, but one in particular jumped off the page. Gum arabic.
Flynn called around town; no one had any. Finally he called his brother, a chemist who works for the Food and Drug Administration in Washington. The brother said gum arabic is a thickener, a food additive. He would ship some to Phoenix.
A week later Flynn sat at his desk and tried to duplicate the techniques Hofmann might have used. He mixed the gum arabic with a new batch of iron gallotannic ink in a glass bottle. He dipped a steel pen into the ink bottle and scratched a sentence across a sheet of century-old paper. The sentence was aged for half an hour with the highest-quality sodium hydroxide. Finally Flynn slipped the sheet under the microscope and focused. The letters were smooth at first, but then began to break in an odd, alligator pattern. They were cracking.
Flynn later figured it out. Gum arabic undergoes a radical change when it is exposed to an alkaline substance such as sodium hydroxide. That change causes the gum to transform from a thin fluid to a material that is thick and brittle. Something that would crack as it dried.
Flynn realized, with some irony, that Hofmann's craftsmanship had betrayed him. He had not made just any iron gallotannic ink; he had mixed a composition that was as close as possible to the ink used in the 19th Century. If Hofmann had ignored the gum arabic, the cracking would never have appeared.
Throckmorton and Flynn found other signs as they studied the Hofmann documents: Under ultraviolet light some documents showed a telltale feathering effect, a running of the ink that suggested that the sheets had been hung to dry. Other tests indicated that more than one ink had been used on some papers.
But after the cracking, they were sure. Flynn and Throckmorton had found a master forger; so good that a half dozen of the best experts in the country had been fooled. They had also found a motive for murder.