Editing Wood (section)

===Specific gravity===
The single most revealing property of wood as an indicator of wood quality is [[Relative density|specific gravity]] (Timell 1986),<ref name="Timell-1986">Timell, T.E. 1986. Compression wood in gymnosperms. Springer-Verlag, Berlin. 2150 p.</ref> as both pulp yield and lumber strength are determined by it. Specific gravity is the ratio of the mass of a substance to the mass of an equal volume of water; density is the ratio of a mass of a quantity of a substance to the volume of that quantity and is expressed in mass per unit substance, e.g., grams per milliliter (g/cm<sup>3</sup> or g/ml). The terms are essentially equivalent as long as the metric system is used. Upon drying, wood shrinks and its density increases. Minimum values are associated with green (water-saturated) wood and are referred to as ''basic specific gravity'' (Timell 1986).<ref name="Timell-1986" />

The U.S. [[Forest Products Laboratory]] lists a variety of ways to define specific gravity (G) and density (ρ) for wood:<ref>{{Cite web |title=Wood Handbook: Chapter 4: Moisture Relations and Physical Properties of Wood |url=https://www.fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapter_04_fpl_gtr282.pdf |publisher=U.S. Forest Products Laboratory |access-date=September 10, 2023 |archive-date=December 30, 2023 |archive-url=https://web.archive.org/web/20231230131945/https://www.fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapter_04_fpl_gtr282.pdf |url-status=live }}</ref>
{| class="wikitable"
!Symbol
!Mass basis
!Volume basis
|-
|G<sub>0</sub>
|Ovendry
|Ovendry
|-
|G<sub>b</sub> (basic)
|Ovendry
|Green
|-
|G<sub>12</sub>
|Ovendry
|12% MC
|-
|G<sub>x</sub>
|Ovendry
|x% MC
|-
|ρ<sub>0</sub>
|Ovendry
|Ovendry
|-
|ρ<sub>12</sub>
|12% MC
|12% MC
|-
|ρ<sub>x</sub>
|x% MC
|x% MC
|}
The FPL has adopted G<sub>b</sub> and G<sub>12</sub> for specific gravity, in accordance with the [[ASTM International|ASTM]] D2555<ref>{{Cite web |title=Standard Practice for Establishing Clear Wood Strength Values |url=https://www.astm.org/d2555-17a.html |access-date=2023-09-09 |website=www.astm.org |language=en |archive-date=April 1, 2023 |archive-url=https://web.archive.org/web/20230401232343/https://www.astm.org/d2555-17a.html |url-status=live }}</ref> standard. These are scientifically useful, but don't represent any condition that could physically occur. The FPL Wood Handbook also provides formulas for approximately converting any of these measurements to any other.