After Niépce's death in 1833, his son, Isidore, inherited rights in the contract and a new version was drawn up between Daguerre and Isidore. Isidore signed the document admitting that the old process had been improved to the limits that were possible and that a new process that would bear Daguerre's name alone was sixty to eighty times as rapid as the old asphalt (bitumen) one his father had invented. This was the daguerreotype process that used iodized silvered plates and was developed with mercury fumes.