Why Flotation?

Hi everyone,

Jacob previously wrote about the process of flotation so I have decided to write about the purpose of flotation analysis. He covered the ‘how?’ of flotation, and this post is about the ‘why?’

What is the purpose of flotation? The stated goal is to intensively investigate much smaller artifacts and ecofacts and, in so doing, form a more comprehensive profile of the excavation unit that supplements the larger artifacts obtained during regular excavation and screening.

Why not just look at a soil sample directly from the site instead of giving it a spa treatment? Or why not use flotation for ALL the soil excavated at the site? The issue is one of scale and limited resources. Sure, it’s possible to take a soil sample and go through it one grain at a time, but it takes a looong time. Likewise, running EVERY bucket of dirt through the flotation process would take too long—there’s only one flotation machine at WMU, and it’s such a process that it’s not even used every year. Flotation occupies a middle ground between the wet screening with 1/8” mesh we routinely do during excavation and examining every grain with a microscope. It can be a hassle to some, but it covers smaller size categories that we would otherwise have no information about, so it’s worth doing!

The mechanism of flotation is twofold. First, it functions as a wet screen with a very fine mesh size. This keeps virtually all the artifacts and ecofacts visible to the unaided eye, while discarding fine sediments like silt and clay that form the bulk of the brown stuff we see when looking at dirt (not that there’s nothing to learn at the microscopic level – for example, pollen grains often preserve extremely well, and can tell us fascinating information about what kinds of plants were present at the site – what crops were grown, what the climate was like, and even how heavily forested the area was. The study of pollen in this context is part of a field called palynology).

This sample reduction is helpful enough, but the special “bubbling” effect Jacob mentioned in his post helps to separate the sample into two portions that are less dense and more dense than water. Typically, things that are less dense are organic and things that are more dense are inorganic; however, some organics are more dense (like bones) and some end up in both (like charcoal). The inquiring mind may wonder if there are artifacts that are neutrally buoyant with densities close to that of water, neither floating nor sinking, and the answer is: yes! However, the most common example is...plastic. At our site, it would be a modern contamination of the historic strata, since people of the 18th century didn’t have plastics yet. It’s not a problem if it misses collection in the flotation screens and drains out with the water.

Removing fine sediment particles and dividing the sample into light and heavy fractions reduces the amount of work needed to process the sample enormously, but going through what’s left is still a heft challenge. Fortunately, it comes with benefits, like getting to investigate different artifact categories that don’t normally show up in wet screening. 

Baked clay can be very hard, like ceramic, or it can be soft and crumbly depending on the firing technique and preservation. The second kind doesn’t hold up well when screening, as the vigorous shaking and rubbing can easily cause it to break apart and fall through the mesh (and the pieces are usually pretty small to begin with, and can look a lot like dirt in the screen to new field school students). At Fort St. Joseph, we’ve found the remains of buildings that were built with materials that include baked clay or daub. It was used to fill in spaces between wooden beams and to build up chimneys (Hartley & Nassaney, 2019, p.95). 

Baked clay does have one particularly interesting feature: sometimes, it can be magnetic. Clay and mud can have tiny grains of iron-containing minerals which aren’t normally enough to respond to a magnet, but can suddenly develop a magnetic field after being exposed to high heat temperatures (Renfrew & Bahn, 2018, p. 89). At a fundamental level, iron (and nickel and cobalt) exhibit a property called ferromagnetism. Ferromagnetic materials have randomly oriented microscopic features called magnetic domains, which respond to a magnetic field. If there are enough of them, like in a solid piece of iron, they can stick to a magnet. In a regular piece of clay, the amount of iron is small enough and spread out far enough that it doesn’t respond to a magnet. However! If the clay is heated to a high enough temperature (like in a fire), the iron grains lose their ferromagnetic properties. This is called the “Curie point” and for iron it occurs at 770 degrees Celsius (Griffiths, 2013, p. 291). When it cools back down, the magnetic domains of the iron particles “freeze” in the direction of the Earth’s magnetic field and can exhibit a strong enough response that they can be picked up by a magnet in the lab.

All this is the “why” of flotation and takes up much more time than the actual bubbling, but it provides evidence from the site that we’d never see otherwise.

Thanks for reading,

Luke

References:

Griffiths, D. J. (2013). Introduction to Electrodynamics (4th ed.). Pearson.

Hartley, E. K., & Nassaney, M. S. (2019). Architectural remains at Fort St. Joseph. In M. S. Nassaney (Ed.), Fort St. Jospeh revealed: The historical archaeology of a fur trading post (pp. 79-100). University Press of Florida.

Renfrew, C., & Bahn, P. G. (2018). Archaeology Essentials: Theories, Methods and Practice (4th ed.). Thames & Hudson.