( Note that because the ink is transferred twice, not once, as in letterpress, an offset plate is positive and right reading, like the original repro and the final printed page.)
Due to the inconvenience and expense of lithographic stones, the stone was eventually replaced with steel plates. These plates were lighter, cheaper, and generally a more manageable printing implement. The first experimentation of using plate substitutes, done by Senefelder himself, was done using zinc. These zinc (and later aluminum) plates were promoted in America as early as 1849 as the ideal medium for reproducing simple outline drawings. It was during the 1870's that the gradual transition from stone to zinc began in America. These rotary machines simply retained the cylinder and the delivery apparatus of the flat-bed machine, while replacing the stone bed and ink slab by a second cylinder, which is called the blanket cylinder. This produced a smooth, quiet-running machine that virtually doubled production. The process today is highly mechanical and automated. There are a variety of inks and a waterless system has been developed