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When trying to figure out if an old map is authentic, the experts at Library and Archives Canada use a number of methods, but mostly they rely on their experience!
By looking at the paper a map is printed on and at the ink that was used, experts can usually get a good sense of when a map was printed and whether or not it's a forgery.
The paper we use today is very different from the paper that was used hundreds of years ago. As new inventions became available, paper got thinner, smoother and brighter. With experience, experts can tell from what time period a certain paper is from simply by looking at it. If a map that is said to be from the 1700s is printed on paper that could only have been available in the 1900s, then they know it's not the real thing! Looking at paper through transmitted light reveals extra clues that can help experts pinpoint when a map may have been made. Most paper-making companies used watermarks showing the company's name and the year the paper was made.
Another technique that map experts use to see if a document is authentic is to closely examine the ink that was used. Just like with paper, inks have changed over the centuries. If an ink from the 1900s is on a map that is supposed to have been printed in the 1600s you can be sure that something fishy is going on.
It's not usual for our experts to find maps that look like documents from one era but are printed on paper and with ink from a more recent time period. This does not mean that they have discovered a fake. Most of the time, it simply means that the map is a facsimile -- a copy of a map that was not made to fool anyone into thinking it was the real thing, but a request from a buyer who wanted to own copy of that map. In the days before photocopiers, it was common practice to order a facsimile of a document.
chainlines: small regular lines that were left behind on paper when it was handmade in a mould made from rows of metal wires or bamboo. Chainlines are spaced farther apart than laidlines.
laidlines: small regular lines left behind on paper when it was handmade in a mould made from rows of metal wires or bamboo. Laidlines appear very close together and run perpendicular to the chainlines.
transmitted light: light placed beneath or behind a document so that the light shines through the paper. The lighting position shows the paper's density and fibers, as well as watermarks, chainlines and laidlines, etc.