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TYLOSES

Tyloses is a waterproof, foam-like substance that forms in the heartwood pores of some species of wood and blocks them, thus impeding or even totally preventing the movement of moisture through the pores. It is an extension of parenchyma cells. It is common in white oak and makes that wood excellent for barrels to hold liquids (tight/wet cooperage) because in the case of white oak, the tyloses completely close the pores and make the wood impermeable even though it is porous. When wood is examined in transverse section, tyloses usually glistens. Woods high in tyloses dry very slowly and are difficult to treat with preservatives for outdoor use.

Other woods that have tyloses include black locust (plentiful), osage orange (plentiful), chestnut (occasional/sporadic) and ash (occasional/sporadic), red oak (infrequent).

Although tyloses is a plural and the singular is tylosis, it can be confusing to use that singular because that term is also a medical term having to do with cancer of the esophagus.

Also, although technically tyloses should be pronounced "tie lows eeze", it is generally pronounced like the singular "tie low sis"

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Examples:

end grain cross sections estimated to be 3/16" x 3/16"
red oak (very little tyloses)	two views of white oak with plentiful tyloses

The pics above are high magnification. In the normal course of woodworking, with nothing more than a 10X hand loupe, tyloses can be harder to see. With my particular type of end grain processing (sanding rather than cutting), it can be even harder, BUT ... there's a trick to identifying it. Tyloses, as you can see in the white oak above, looks a lot like someone put little glass windows in the pores, then smashed the windows, then glued the pieces back together. Even when the pores are obscured with sawdust, you can usually tell the difference. In the 1/4" square cross sections below at 12X, the first two show red oak with pores that are either open or look like smooth blobs. The second two show white oak with pores that clearly are NOT smooth blobs but rather have the "broken window" look.