Friday, January 8, 2010

Screen it!

Once you have shoveled and troweled, what are you going to do with all that soil? Why, screen it of course! An archaeological investigation is not possible without the use of a screen for sifting away soil to find artifacts. Screens can vary in two main ways: set-up, and size of the mesh.

How the size of the mesh can change is self-explanatory...either the mesh is made up of smaller holes or larger ones, depending on the site and what materials the excavator is looking to investigate. We use a quarter-inch mesh, which is perfect for catching little bits of glass and ceramic, while letting the really, really tiny pieces pass through the screen. Of course, this mesh can also be made of different grades of material, though the stronger mesh is best for extending the life of a screen. There is more variety in the types of screen available, some of which are the free-standing screen, the tri-pod screen, the hand-held screen, the nested screen, and the H-frame. It is possible that you know these screen by different names, but these are the most basic for describing them.

Free-standing or Lavish screens are most people's favorite, except when it comes to being mobile. If you have worked with us down at Port Tobacco, you have had the privilege of using one of the lovely Lavish screens made by Dan Coates. While these are very comfortable to use with multiple people, are lighter than many free-standing screens, and can screen large quantities of soil faster, it is still a bit difficult for one person to lug one across longer distances, and is too bulky for shovel testing (especially when testing in a densely wooded area). Nevertheless, this is definitely one of the easiest screens to use, requiring little effort on the part of the screener.

Tri-pod screens (image to the left) have also been used down at Port Tobacco, and are exactly what they sound like--a screen in a wooden frame suspended by a rope from three rods that form a tri-pod. These require a little more effort than the Lavish screens since you are in charge of shaking the soil rather than having the assistance of wheels and a track, but these screens are conveniently collapsible able for travel. Hand-held screens (image to the right) are just the wooden frame with the wire mesh, and, due to their smaller size, are perfect for sites that need to be hiked into...however, you do have to support the screen while shaking it, and with a heavier clay soil I imagine this can get quite tiring. These screens also are unable to screen large quantities of soil quickly due to their smaller size.

Nested screen (at left) are generally used for finding very small materials. The screens vary in sizes (measured in millimeters) and rest on top of one another. When shaken, the largest material remains on the top screen while everything smaller than the size of the mesh falls through to a second screen which has smaller holes, once again allowing smaller materials to pass through while stopping anything larger than that particular mesh size. Three or four sizes can be stacked on one another, and are tedious to use but excellent for recovering tiny artifacts such as fish scales or very small beads like the one in yesterday's blog.

It is likely that many of you have also used an H-frame screen (image to the right), our screen of choice when out excavating shovel-test pits. This screen rests on two legs and is held up by two handles the screener holds and used to shake the screen. These are perfect for a site that requires mobility, as a person can carry one or two without much difficulty. These screens also fold flat, making it quite easy to fit several of them into Jim's truck.

A tarp can also be used in conjunction with these screens to aid in backfilling, though they are best for filling in shovel-test pits.

So, when searching for artifacts in soil, choose your weapon and just screen it!


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