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Despite its name, oil of guaiac does not come from the Guaiacum tree but from the palo santo tree, Bulnesia sarmientoi. The tree inhabits a part of the Gran Chaco area in South America, around the Argentina-Bolivia-Paraguay border. It belongs to the family Zygophyllaceae. The tree has recently been reclassified as Gonopterodendron sarmientoi. Its wood is often traded as 'Paraguay lignum vitae', since its properties and uses are similar to those of the 'true' lignum vitae trees of genus Guaiacum, which are close relatives. The wood is also traded as 'vera' or 'verawood', names that may refer as well to the more closely related Bulnesia arborea. Palo santo, Spanish for 'holy stick', is a common but rather ambiguous name of Bulnesia sarmientoi. The same name is borne by Bursera graveolens, a tree of the family Burseraceae. The heartwood is brown, black and green, with streaks. The basic specific gravity of the wood is between 0.92 and 1.1 g/cm³. The heartwood varies in colour from light olive green to chocolate brown, and the sapwood is mostly thin and light yellow. The tree is weedy and willowy and is sometimes called ironwood.

Composition and Properties

Oil of guaiac is a yellow to greenish yellow semi-solid mass which melts around 40-50 °C. Once melted, it can be cooled back to room temperature yet remain liquid for a long time. The oil is composed primarily of 42-72% guaiol, bulnesol, δ-bulnesene, β-bulnesene, α-guaiene, guaioxide and β-patchoulene. Guaiol is a sesquiterpenoid alcohol. It is a crystalline solid that melts at 92 °C. The oil is considered non-irritating, non-sensitizing, and non-phototoxic to human skin.

Guaiol, also called champacol, has the formula C₁₅H₂₆O and a molar mass of 222.37 g/mol. It occurs in several plants, especially in the oil of guaiacum and in cypress pine. Treated with electrophilic bromine reagents, it gives a deep purple colour.

Guaiazulene has the formula C₁₅H₁₈ and melts at 31 to 33 °C. This low melting point makes it difficult to handle, unlike the crystalline parent azulene. It is an approved cosmetic colour additive in the United States and a component of some skin care products, alongside soothing compounds such as allantoin.

Storage and Identification

The oil is soluble in alcohol and insoluble in water. It keeps for 24 months or longer when stored in tightly sealed containers in a cool, dry place, protected from heat and light. The oil carries the EC number 289-632-8 and the FEMA number 2534.

Conservation and Uses

Bulnesia sarmientoi was listed as endangered in the 2018 publication of the IUCN Red List, due to the deforestation of Gran Chaco and a strong global demand for its wood, extracts, and essential oils since 2001. The species has been listed in Appendix II of CITES since 2010. The IUCN expects the global population to fall by around 50% over three generations. In 1998 the species was still listed as lower risk and conservation dependent. Palo santo is employed for engraving work and for the making of durable wooden posts. Its resin can be obtained by means of organic solvents and is employed to make varnishes and dark paints. The wood has also been used in indigenous medicine in South America. The wood makes good charcoal and high-quality timber, and its essence is appreciated for skin-healing properties. Despite its density it ignites easily and gives a fragrant smoke. Small pieces of the wood are burned as natural incense in spiritual rituals. Maté cups are carved from it, and a beer is aged in tanks made of the wood. Oil of guaiac was a pre-Renaissance remedy for syphilis.

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