Earlier today, while browsing the wallpaper downloads at the National Geographic site, I came across this Dali-esque photo of tannin-stained waters and sand dunes in Brazil. Enchanted by it's sinuous, almost austere beauty I thought it was worth sharing here. ( In the Amazon Basin of central-eastern Brazil, the many rivers that transect this ecoregion occasionally rise and flood the landscape. These dark tannin laden rivers are all "blackwater," meaning that they are stained by tannins and carry few sediments, and the flooded forests that surround them are known as white-sand igapó forests.)
Now, I know tannin is water soluble and I sort of remembered it causes a problem with some woods and paints but I couldn't quite remember what. (It's Saturday ~ my brain wasn't going anywhere!) At the www.seal-it.ca site, there is a short article 'Tannins and Wood' but it didn't answer the question niggling in the back of my mind. They did in another article, however, raise the issue of alkalis leaching and its impact (that's another post).
I started wondering if tannin was an alkali and found out at Wikipedia that they 'are astringent, bitter plant polyphenols that either bind and precipitate or shrink proteins and various other organic compounds including amino acids and alkaloids.' ~ (aren't you glad you asked?) ~ but they are not alkalis. If you want to know what I found out, feel free ~ http://en.wikipedia.org/wiki/Tannin ~ it's really quite interesting. Nonetheless, I had to go back to noodling about to find what I knew I knew and what my indolent mind was not going to surrender.
So, ready for me to make a long story short?
What I found poking about hither and yon is that many types of wood (such as redwood, red cedar, mahogany, Douglas fir, etc.) contain tannins which tend to migrate to the surface and discolor the paint. Ta da! It was that simple! Which makes sense, don'tcha think ~ chemical substances can migrate out as well as in making a good argument for a protective barrier of a sealant between the wood and the stains or paints used as a surface treatment.
My brain admitted it knew that all along and promptly went back to idling its way through more photos at National Geographic.
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