by Steve Archer
- What is a phytolith?
- “Phytolith” comes from Greek roots phyto (plant) and lithos (stone). “Plant
technically, they are silica casts of plant cells or spaces between cells
in plants. Silica, dissolved in groundwater as monosilicic acid, is
taken up into the plant during its life. The silica is deposited in
the plant as opal silica, chemically identical to the “opal” you
know as a semi-precious stone. These tiny opal particles take
on the shape of cells, or “negative” casts of the space between
cells. Hairs, prickles, and other surface structures of plants also
often become phytoliths.
- Who discovered phytoliths?
- Interestingly, one of the first people to observe and describe phytoliths
under the microscope was Charles Darwin during the voyage of the Beagle—where
he began to form his theories of natural selection and evolution. He
noted the phytoliths while looking under the microscope at dust blown aboard
ship from storms. However, his interest stopped with simple description.
- Archaeological applications of phytoliths began in 1971 with a paper published
by Irwin Rovner, now at North Carolina State University. Major phytolith
discoveries came in the late 1970s with Deborah Pearsall using phytoliths
to document early maize agriculture in Ecuador.
- How do the phytoliths get in the ground?
- Phytoliths are released from the rest of the plant’s structure where
the plant decays or is burned. This can be either where a plant naturally
died and decayed, where humans have disposed of plant residue, or even where
animals have eaten plants and deposited their dung, as phytoliths survive
the digestive process.
- Do all plants have phytoliths?
- No. Phytolith production in plants follows evolutionary groupings. Some
plants, particularly grasses and other monocots produce many phytoliths, and
in fact use them as part of their support structure. Other plants do
not produce phytoliths at all. One plant family of particular interest
in Virginia, the Solanaceae, does not produce phytoliths. This means
that plants in this family, such as tomatoes, potatoes, and, importantly,
tobacco, cannot be seen archaeologically using phytoliths.
- However, some plant families, such as the grasses (Poaceae)
have phytoliths even more diagnostic, or taxonomically specific, than seeds
or pollen. This
is important because particular grasses have different growth environments,
and different geographic origins. This is highly useful in understanding
a site’s environment and landscape.
- Why study phytoliths?
- Phytolith analysis is only one component of a broader sub-field of archaeology
called paleoethnobotany or archaeobotany,
the study of plant remains in archaeology. Prior to the twentieth century,
most material culture was made of plants. Think about a world without
construction wood, plant foods, plant medicines, wood furniture, trees, ornamental
plants—and you can begin to see the importance of looking at plants
to understand the past. Other types of archaeobotanical studies include
macrobotanical analysis (the study of charred plant materials such as wood
and seeds recovered from flotation), palynology (the study of pollen and spores),
and a variety of other microscopic and chemical specialties. At Colonial
Williamsburg, we do macrobotanical and phytolith work in-house, and have done
other kinds of specialized archaeobotanical studies in conjunction with researchers
- Each archaeobotanical “data set” gives slightly different information,
and the best understanding of the past is obtained when more than one type
of data is used.
- Phytoliths are of particular interest to archaeologists
because they are highly resistant
to decay, more so than most other types of archaeological plant remains. Because
they are essentially stone, they preserve indefinitely in all but the most
extremely alkaline burial conditions.
- Are phytoliths the same as pollen? I’ve read about pollen
studies in archaeology.
- No. Phytoliths are also microscopic plant remains, but the similarity
ends there. Pollen studies have a much longer tradition in archaeological
research, and also tell you different things about archaeological sites and
past environments. Pollen grains are the male reproductive gametes that
fertilize female flowers. They have a tough outer structure that permits
them to survive for long periods of time in certain depositional settings.
Pollen tends to preserve very poorly on most archaeological sites, and
it is often difficult to understand its “formation process” (i.e.,
how did this pollen get into this deposit, and does it represent a plant
used during the actual occupation of the site). Many palynologists
tend to focus on stratified lake sediments to understand environmental
change over a long period of time. Phytoliths can also be used in this
manner, and scientists are beginning to look at phytoliths and pollen together
in lake and other natural deposits.
- We don’t currently have the capacity to extract or interpret pollen
in-house, and that work is generally done in collaboration with other researchers.
- Can phytoliths tell exactly what plant or kinds of plants were used
in a garden feature?
- Short answer, no, with a few exceptions. One thing that is difficult
to comprehend—until you’ve spent some time with it—is the
sheer complexity of archaeological deposits. Today there are virtually
limitless kinds of tests and extractions that generate “data”
from “dirt.” We
now have the potential to do more “data
production” with a cup of soil than an archaeologist in the 1950s
could with an entire
site. The trick is making good choices in research design, and then
figuring out what it all means!
- Remember that not all plants produce phytoliths, first
of all. That eliminates many possibilities. Second, think about how
a garden is actually used. Soil is moved around and brought in from
other places. Plants
are continuously harvested and planted, with different plants in the same
location in different seasons or years. Some, like pumpkin vines, might
be plowed into the soil before a next planting. Others, like carrots,
might be completely removed from the soil to consume, leaving no trace. Then,
think about what goes into garden soil. Fertilizer, which may contain
animal manure—and consequently phytolith traces of all the plants
the animal ate! Or yard compost containing many other plants from different
areas of the site. Now compound these processes over twenty or a hundred
years. What plant grew in that soil? Probably many, and we will
get traces of some of them, plus traces of all these other kinds of plants.
It is important to point out that archaeological sites are dynamic rather
than static, and the problem of “what plant grew there” is
usually a problem with the question, rather than the phytoliths.
- That being said, some plants that leave very distinct
and numerous traces,
like maize (corn), for example, often give a strong suggestion that a certain
area may have been used to grow corn (or squash, etc.), especially if that
use remained relatively unchanged for long periods of time.
- So, what use are phytoliths if they can’t exactly reconstruct
- We interpret phytoliths as assemblages and in comparison with other
samples from the same or different sites. Looking at the overall proportions
of phytolith types within a sample (we generally count hundreds of phytoliths
from each sample)—we can ask different, and broader kinds of questions. How
was this yard space used? How did the site environment change over time?
Where did people process their corn? Where was an orchard located? Was
this lawn maintained, or was it allowed to go to pasture? What areas
of the site were used repeatedly for the same purpose, and what areas changed
- It is important to develop our questions in conjunction
with both what we know, and what we want to know about a site or group
of sites. What is our understanding of the level of disturbance of a site
or deposit? If
the site is primarily industrial, should we ask foodways questions of this
- These questions are more about how people interacted
with their landscape, and in many ways are more significant and meaningful
than “did this
garden contain bouncing bet, or johnny jump-up?” As with other
artifacts, good phytolith questions further archaeological interpretation—“what
was life like in the past?”—rather than being an end in and of
- How big are phytoliths?
- Phytoliths range between 5 and 200 microns, most being around 10-30 microns.
(A micron is 1/1000th of a millimeter.) A human hair, for comparison, is approximately
100 microns in width.
- Can I see phytoliths with the naked eye?
- No.We use a transmitted light microscope, with magnification between
100X and 1000X to see them. But, you certainly have felt phytoliths
at one point or another. If you’ve ever put a blade of grass between
your lips and felt the roughness, or cut your bare arms walking through a
cornfield, that is due to the silica phytoliths in these plants.
- How do you get phytoliths out of soil?
- The short answer is, through a combination of sieving with water, gravity
sedimentation, acid organic removal, and heavy liquid flotation. The
whole process from raw soil to phytolith extract can take up to a month to
- In slightly more detail, this is the process:
- 1. Excavators in the field take small bags of soil from
many contexts on a site. The
archaeologists work together to decide on a research plan for the site, and
how the phytolith analysis may contribute to the overall research goals.
- We use about 80 grams of soil, usually this amounts to
about a half cup. The
raw soil is first deflocculated. That is, it is stirred
in water over several days with detergent (we prefer Cascade) to break up
the lumps and aggregated soil. The goal is to have all of the individual
soil particles ionically repelling each other to facilitate the next step.
- 2. The soil is then sieved with water
using very fine-meshed screens of 250 and 53 microns. All soil larger
than 250 microns (phytolith sized) is discarded. Soil between the sizes
of 53 and 250 microns (the “C fraction”) is saved for special
studies of larger sized phytoliths.
- 3. Dissolved soil in water that is smaller
than 53 microns is collected and
allowed to settle. At this point, the material larger than phytoliths
is gone, and we need to remove clay particles smaller than phytoliths (less
than 2 microns) that impair phytolith extraction and viewing.
- 4. To remove clays, the sediment, dissolved
in water, is allowed to settle in an 8 cm column of water for one hour. At
this point, clay particles are still in suspension, while phytolith-sized
particles have settled to the bottom. The water containing clays is
poured off, saving the settled soil at the bottom of the beaker. This
process is repeated many times until the supernatant water is clear.
- 5. The soil now has particles both larger and and smaller
than phytoliths removed. At
this point, organic chemicals (humic colloids) in the soil need to be removed
from the sediment. Organic removal is accomplished
by heating the soil in a combination of nitric acid and potassium chlorate,
sometimes called Schulze solution. This is done in the fume hood on
a hot plate.
- 6. The sediment is then rinsed clean of the acid, using
the centrifuge and fresh water.
- 7. Phytoliths have a slightly lower specific gravity
than other components of the remaining soil, such as quartz particles. To
separate the phytoliths from the sediment, we prepare a heavy specific gravity
liquid using a chemical called sodium polytungstate. The process is
like making a “black
and tan” beer, or the separation of oil and vinegar in a salad dressing
cruet. By adding the sodium polytungstate solution to the test tube
and centrifuging, the phytoliths float on top of this liquid, while other
minerals remain in the bottom of the test tube. The phytoliths
are collected using a pipette (like an eyedropper), and placed into new test
- 8. The phytoliths are rinsed clean of any remaining chemicals
using water and acetone, and are then ready to be put onto a slide and
- How are phytoliths counted?
- An analyst systematically counts the slide by moving the field of view across
the microscope slide in lines (transects). He or she notes, counts and
measures different known types of phytoliths, as well as recording new unknown
phytoliths with drawings and photographs. We aim to count a statistically
useful number of known phytolith forms, usually 200 per slide.
- How are phytoliths identified?
- In short, how do you know what you’re looking at? Phytolith research
has been a component of archaeology since 1971. Botanists also have
been documenting phytoliths in various plant families, so much is known about
certain forms that are seen over and over again. In particular, different
sub-families of grasses are well studied, as are some major crops like maize
and cereals. These have been thoroughly documented in scientific literature.
(Search the collection on-line)
- Our second component to identifying phytoliths is developing
and maintaining a phytolith reference collection of plants likely to be
found in our archaeological samples. Using herbarium and other known specimens,
these plants are processed to see if they produce phytoliths. We maintain
a computer database of over 1000 specimens, including microsocope photographs
of phytoliths found in them. This is the only mid-Atlantic historical
archaeology phytolith research collection.
- How do you interpret phytoliths?
- Phytoliths in one sense are like any other kind of artifact. They
do not speak for themselves. Each phytolith analysis is a project with
a research design. That is, we choose specific contexts on the site
to analyze, because we are interested in a particular problem. You
don’t just “do” phytolith analysis to see what it says,
just as a list of artifacts alone is not informative about a site. Examples
of questions we’ve addressed with phytoliths are: How was the Tucker
garden managed and maintained? Are there traces of seventeenth-century landscapes
still visible in plowzone at the Rich Neck Plantation? How do a series
of fills inform us about the landscape of the Wray site during and after its
- Creating a story based on phytolith evidence entails knowledge
of both the
archaeology of a site and the botanical aspects of the plants
- As with all other aspects of the archaeological program here,
the more analysis
we do, the more refined and better our interpretations become, by having the
ability to make more comparisons, as well as clarifying unidentified phytolith
- What places has the phytolith lab done work on?
- In and around Williamsburg: St. George Tucker Garden; Carter’s Grove;
Rich Neck Plantation; Yorktown; Jamestown. Elsewhere on the East Coast:
Monticello; Charleston, South Carolina; Thomas Jefferson’s
home at Poplar Forest.
- Who started our phytolith lab?
- In 1995, the Department of Archaeological Research funded the creation of
the phytolith lab, under the direction of Dr. Lisa Kealhofer. Dr. Kealhofer
took a position at Santa Clara University in 1999. Steve Archer, a Ph.D.
candidate at the University of California at Berkeley, is currently in charge
of the phytolith laboratory.