Gelatin dry plates were is use from roughly 1880 to as late as 1975.  The plates were machine coated, with even coating at the edges, the edges were cut rather than ground.  These plates are occasionally varnished and tend to have a neutral grey to black image tone (sometimes they look vaguely purple).
In contrast, wet plates date from 1851 to roughly 1880.  The plates were coated by hand, making the coating often uneven at the edges, and the edges of the glass were often ground rather than cut. 

We received some glass plate negatives from a customer that were in really bad condition.  The plates were stored in an attic and were the victims of a house fire.  Heat, smoke and water combined to try and ruin the plates.  When we received them, they were stuck together and filthy.

Typically a soft brush is used to gently brush dust and loose dirt from the emulsion side of the plate, distilled water and cotton balls are then used to clean the back of the plate.  If there are any signs of flaking or lifting of the emulsion, brushing is immediately halted.  Due to the way they were stored, stacked together, and the effect of the water causing them to be glued together, we had to take special care in separating and cleaning them. 

Using clean filtered water and Kodak Photoflo we were able to re-wet the emulsion and carefully separate the plates. After cleaning the plates were allowed to carefully dry and then scanned to digitally preserve the images.

(if you look at the above image and let your eyes go out of focus, a third image will show up in the middle of these two and it will be in 3d!)

Once cleaned, dried and scanned, the plates are stored vertically and individually in acid-free envelopes, in an acid-free box, in a cool dry climate. They should never be stored in an attic or basement or be subjected to extremes in temperature.

We also received a glass plate from the early 20^th century featuring an image of some brass cars in front of the United First Parish Church in Quincy, MA.

When the plate came to us it was coated in about 100 years’ worth of dust and dirt. 

Cleaning a glass plate helps in preserving it.  Removing the dirt minimizes the chances of further image deterioration and allows us to make the best possible scan of the plate.  Using water to wash glass plates isn’t recommended except under dire conditions. We DO NOT recommend that you try and clean glass plates yourself, this should never be attempted by someone without years of experience in handling these priceless negatives.

View the images here.