At the press conference that Dagny Taggart and Hank Rearden gave announcing the opening of the John Galt/Rio Norte Line, Rearden stated that the formula was secret:
"Inasmuch as the formula of Rearden Metal is my own personal secret, and in view of the fact that the Metal costs much less to produce than you boys can imagine, I expect to skin the public to the tune of a profit of twenty-five per cent in the next few years."
"What do you mean, skin the public, Mr. Rearden?" asked the boy. "If it's true, as I've read in your ads, that your Metal will last three times longer than any other and at half the price, wouldn't the public be getting a bargain?"
"Oh, have you noticed that?" said Rearden.
However, earlier in the book, Dagny and Jim were arguing about whether to order the rails for the Rio Norte/John Galt Line from Orren Boyle or Hank Rearden, and about whether they should be made out of steel or Rearden Metal. Dagny claimed to have seen the formula for Rearden Metal:
"Then what on earth do you know about Rearden Metal?"
"That it's the greatest thing ever put on the market."
"Because it's tougher than steel, cheaper than steall and will outlast any hunk of metal in existence."
"But who says so?"
"Jim, I studied engineering in college. When I see things, I see them."
"What did you see?"
"Rearden's formula and the tests he showed me."
Presumably, too, his research staff would've known about it, as would his Mills Superintendant.
That being the case, why did Hank Rearden call the formula his "personal secret"?