If you are over the age of 20, you probably received dittos as handouts in school. You should remember them: Purple ink, sometimes rather faded and hard to read. Sometimes they'd even have this chemical smell to them. It's not something that you think about often; it's the sorta thing that you don't see any more and think "Oh! I remember that!" when somebody mentions it. In any case, those handouts probably came out of a ditto machine.

Ditto machines were used for making copies of documents long before Xerox even existed. They were also less expensive per-copy than machines using the Xerox copying process for a long time.

Here's how a ditto machine works:

A master sheet is taken and loaded into a typewriter or placed onto your desk. This sheet works somewhat like carbon paper. There are two layers. One layer is a sheet covered with a thick layer of purple ink. The other layer is a very thin sheet pressed against this layer of purple ink. When you write, or type, on the master sheet, the ink is transferred from the first sheet to the second, much like the way carbon paper works.

The second sheet is then loaded into the machine, onto a special drum. The drum is coated with a solvent that dissolves the ink very slightly. Sheets of paper are fed between the drum and a roller, and with each sheet that goes through, a small amount of ink is transferred from the drum to the paper. This solvent, which contains methanol as a major component, is that chemical smell that you tend to notice on freshly run off dittos.

The process is incredibly simple; it does not require electric motors nor fancy lenses. It is entirely mechanical. this is why ditto machines are so incredibly reliable.

The primary problem with ditto machines is that you can only make 500 or so copies before having to create a new master. Not only that, but the last copies you make will look considerably lighter and harder to read than the first ones. You just can't get enough ink transferred to the master sheet to make many copies. Additionally, the purple ink used in the process would frequently get all over your clothing and make quite the mess.

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