Wood
Analysis Basics
There
are many questions that come to mind when viewing or studying objects that are
made from wood. Where did the object come from? What kind of wood is it made of?
Why was this type of wood used? What is the tradition, with respect to wood
use, of the culture producing the object? Many of these questions can sometimes
be answered by a microscopic examination of cellular structure of the wood.
Microscopic wood anatomy is a scientific endeavor with a long, famous history. One of the earliest materials used for microscopic observation by Antony van Leeuwenhoek (1632-1723) was bark from a tree. The most famous use of wood anatomy and identification in forensics was the work of Arthur Koehler at Forest Products Laboratory, whose analysis of a ladder used in the Lindbergh Kidnapping led to the apprehension and conviction of Bruno Hauptman in “The Crime of the Century” in the 1930’s. A popular current use of microscopic wood anatomy and identification is to help authenticate antiques, as seen on Antiques Road Show.
Why would anyone want to know what type of wood has been used in an object?
Microscopic
wood identification and analysis from objects d’art have limitations not always
apparent to conservators, curators, art historians and collectors. These limitations are based on many factors,
including the size of the sample, the level that one looks at the sample (i.e.
microscopic) and the evolutionary conservative nature of wood with respect to
species determination. The hierarchy of
plants is based on a taxonomic system developed by Carl Linnaeus (1707-1778),
where each specific type of plant is given a binomial (“two-names”) the genus
(like our family names, e.g. Smith) and a species name (like our first names,
e.g. John). The binomial is italicized, with the genus first and the species
epithet second (e.g. Eastern White Pine is Pinus strobus). When one
refers to an unknown species the term “sp.” is used and when one is referring
to more than one species in a genus, the plural “spp.” is used. Similar genera
are grouped into Families (ending in –aceae), similar families into Orders,
similar orders into Classes, and similar classes into Divisions.
Determination of Genus
When
people think of types of wood, they generally are thinking in terms Pine, Oak,
Maple, Spruce, etc., which describe genera of trees (Pinus, Quercus,
Acer and Picea, respectively). Even with small samples,
identification of wood to the genus level is usually very accurate.
Species Groups
Species
groups are groups of species, within a genus, which have anatomical similarity.
For instance, the Pines (Pinus spp.) can be grouped into the Red Pine
Group, White Pine Group and Yellow Pine Group. The Oaks (Quercus spp.)
contain the Red (Erythrobalanus) and White (Leucobalanus) Groups and the
Maples (Acer spp.) have species
that separate into the Hard and Soft Groups.
Species Determinations
The
microscopical determination of wood to the species level is usually not
possible. These limitations are based on the evolutionary conservative nature
of wood. Species of wood (trees, and
plants in general) have been determined in the past from the number, size,
shape and orientation of external features, such as reproductive structures
(flowers, fruits, cones), foliar structures (leaves or needles) or other parts
of the plant like bark or branch morphology.
These characters are rarely present in objects made of wood. Current, molecular techniques of DNA
analysis are moot because of the paucity or lack of DNA in wood.
Species
determinations are empirically (straight from the anatomical characters,
without assumptions) possible for a few taxa. In the Walnut/Butternut Group (Juglans
spp. Juglandaceae), American Black Walnut (J. nigra) can be separated
from English/European/Persian Walnut (J. regia) by the presence of short
chains (1-5) of calcium oxalate crystals in the axial parenchyma and irregular
spiral thickenings in the vessels termed “gashes”. Some empirical species separations are conditional in nature. For
example, in True Mahogany (Swietenia spp. Meliaceae), if the specific
gravity (density) of the wood is above 0.65, then the wood is Cuban Mahogany (S.
mahogani) and not Honduran Mahogany (S. macrophylla). If the specific
gravity is below 0.65, either species may be present.
Some
species determinations are deductive through geography (and other ways). In the
genus Liriodendron (Magnoliaceae), there are two species worldwide,
Tulip Poplar (L. tulipifera), native to the United States and Chinese
Tulip Tree (L. chinensis), native to China. If the wood in question is
in a colonial American object, then it is deduced to be Tulip Poplar. The True
Hickories would exemplify a chronological deduction. This genus had a distribution
across the northern hemisphere prior to the Pleistocene (Ice Age), but
afterward was restricted to eastern Asia and North America. Its presence in
colonial objects is deduced as being American.
The
assignment of provenance (where the object came from) based solely on
microscopical wood identification is, empirically (without making any
assumptions) virtually impossible, because of the limitations mentioned above.
Most groups (genera, species groups) show cosmopolitan distributions, i.e.,
there are species on both sides of the Atlantic or Pacific. However, a few
genera or species have very limited natural distributions and are good
“indicator” woods. For example, Capá Blanco (Petitia spp. Verbenaceae)
is composed of only one species, native to the Caribbean Islands. Its presence
in an object would indicate that it originated in the Caribbean.
Wood
identification does not, by itself, determine provenance of objects, but is an
element used along with constructional and stylistic features. The determination of provenance of objects
d’art has many other complicating factors.
Complicating Factors
Common Names
Most
times, woods are referred to by their common names; Pines for the genus Pinus,
Oaks for the genus Quercus, etc.
In general, this scheme works well, but there are also many times when
confusion can occur with the use of common names. For example, the name
“sycamore” is the genus Platanus in the US, but in England and Europe it
is a species of Maple (Acer pseudoplatanus). Also the term “poplar” can
represent the Tulip Poplar (Liriodendron tulipifera), a wood common in
colonial furniture or the True Poplars (Populus spp.), a wood that
appears in European furniture.
The
Linnaean binomial is the preferred term used when conversing about wood, unless
a well accepted trade name (the most common “common name”) is assumed. In
addition to the above confusion, some individual species of plants, trees or
woods can have numerous common names, while a particular common name can have numerous
species associated with it. For example, there are at least 135 common names
for “rosewood”, 446 common names for “mahogany” and 475 common names for
“cedar”. A reasonable estimate of the
total number of recognized common names for wood approaches 170,000.
Commercial vs. All Woods
Another
complicating factor of wood identification is that most texts and computer
software include only “commercial” species and neglect species with limited
distributions or have woods that are produced in low volumes for commercial
markets. This may be well and good for identification of lumber, but many
ethnographic or “primitive” objects as well as those of small size may be
composed of local trees or shrubs that do not appear in commercial markets. For
example, there are about two dozen commercial woods used in colonial furniture,
whereas for “primitive” furniture or ethnographic objects of unknown origin,
any of the 27,000+ species of trees and shrubs could have been used.
Species Introductions
Since
the 16th Century, trees have been imported into the British Isles
and elsewhere as novel horticultural species and as plantation crops. For
example, Eastern White Pine (Pinus strobus) was imported to England in
the mid 16th Century as a horticultural curiosity, while American
Black Walnut (Juglans nigra) and Persian Walnut (Juglans regia)
were imported in the 17th century as food crops. Juglans regia
later became known as English Walnut, but is originally from the Middle East
(Persia).
Importation of Wood
During
the past, many fragile objects that were transported across the oceans by boat
were packed in wooden crates. Upon arrival at their destination, the crate wood
may have been reused for other objects, rather than as firewood. Thus something
like Scots Pine (Pinus sylvestris – Red Pine Group), an indicator of
English/European provenance, may have ended up in American furniture. Also,
during the colonial period, the British Navy exclusively used whole trees of
Eastern White Pine (Pinus strobus) as ship masts. When these masts were
damaged, the wood may have been used in objects (large and small) made in
England. In addition, tropical woods (True Mahogany, Rosewood, Pauduk, etc.)
were imported into Europe by the Dutch (Dutch East India Company) and English
as early as the 17th Century.