The Flotation Technique in Archaeology

Posted on May 3, 2012


Flotation has always been somewhat of a mystery to me.  My base understanding of what flotation entails is that you use water and sieves to separate archaeological materials from soil. As we all know certain objects are heavier than others, wood and seeds are light and thus float, stones are heavier and sink . By adding water and separating these materials from the heavier soil, the lighter materials float to the surface.

I have never seen this technique in person but have been involved in analyzing flotation material for the last four months.  This has sparked my interest about this less well-known archaeological retrieval technique that I would like to know more about.  Flotation is not widely used in archaeology in South Africa because we are not taught the technique and Archaeobotany is not widely practiced here. The site I am currently working on could set a precedent for the use of this technique if what is found to be worth it.

If there is an archaeologist out there that has used this method and would like to tell us more, discuss why they used it and if it was worth it, please contact me – maybe we can do a follow up interview and explore flotation even more.

Where did flotation start?

In 1960 Dr. Hugh Cutler took a sample of soil from an excavation and poured it into water, charred seeds floated to the surface.  These would probably have not been found through the normal archaeological sifting processes.   Since this event experimentation with flotation grew and became mechanized (Struever 1968:353).

Students retrieving charred plant remains by flotation. (Photograph courtesy of Research Laboratories of Archaeology. More about the photograph) – Source

The aim?

The aim of flotation is to “recover animal bone, seeds, and other small cultural remains lost in the normal screening of soils from archaeological sites” (Struever 1968:353).  As I understand this, faunal (animal) and floral (plant) materials is trapped in soil, and is sometimes hard to distinguish in a dry sieve. When one loosens the soil, with the help of water these materials separate from the soil and suspends in the water.

Some materials are so small that most mesh sieves holes are too big. The standard mesh is 1mm of mosquito net, and although this is the norm, sometimes seeds and other small materials are overlooked. Floating of the material that has already been sieved might also yield some materials missed, for example seeds.

Credit: James King-Holmes/Science Photo LibrarySource

The process:

This brings us back to the density of the objects as discussed in my introduction paragraph. Seeds which are light will float to the top, bone and wood is slightly more dens and will be suspended in the middle.

Imagine a clear glass of water into which you drop a ball of mud that has pieces of grass, leaves and other materials stuck to it. If you stir it a bit and let it settle, the heavier soil will sink and the leaves, grass etc will float to the top. The water will look darker at the bottom and be clearer as you move upwards. The lightest materials will float at the top and some other materials will float just below the surface. It all depends what is in your ball of mud and their different weight/densities.

Before a bag of soil goes through the flotation technique each bag has to be weighed and correctly documented (Schock 1971:230). You can see some of this being done on the videos below as well as the technique in action.

Bucket Flotation:

Flotation using a machine:

A-Z of Archaeology: ‘S – Seeds (Botanical Remains)


The floated remains are then left to dry and can then be sorted. Most archaeologists will lay out all the finds from one bag onto one tray. This tray will then be sorted and sent to be analyzed by the various specialists, such as archaeozoologist or archaeobotanist.  Each specialist will analyse the material according to the standards of their disciplines i.e. the Archaeozoologist will look at each piece of faunal  material, document and weigh them and then write a report about their findings. The materials once analyzed may yield information about diet and other aspects of a culture such as living conditions, gathering patterns and environmental conditions i.e. seasonality of site occupation which will in turn provide a greater insight into the daily life of a culture.

Seeds recovered by flotation from EeRb 140 feature.
(M. Wollstonecroft photo)

A note on drying:

“It will often take a couple days for charred material collected by water flotation to dry. Some of the dried material may have to be refloated or rinsed to get rid of small bits of dirt or clay. This material will again have to be thoroughly dried before sorting for small seeds is feasible.” (Schock 1971:230) “Plastic serving trays and empty film reel holders make handy containers for drying.” (Schock 1971:230)

Please note: Do not EVER put any materials that are dying in direct sunlight. It damages the materials and if anything dries too fast it might crack or deteriorate rapidly.

Credit: James King-Holmes/Science Photo LibrarySource

I hope that this journey through flotation has been as enlightening for you as it has been for me.

If you are interested in Archaeobotany this is a great blog to follow:


Schock, J. M. (1971). Indoor Water Flotation – A Technique For The Recovery Of Archaeological Materials. The Plains Anthropologist, 16(53), 228-231.

Struever, S. (1968). Flotation Techniques for the Recovery of Small-Scale Archaeological Remains. American Archaeology, 33(3), 353-362.