Quick Topic for Forestry: cork

August 05, 2012


The forest product worth it's weight in gold when you need it and instantly forgettable once it is taken from the bottle, cork.  Have this topic ready for one of those class periods when there's no time to start a new lesson and too much time to safely bet the principal won't walk in before the bell.  Hopefully you have enough FWW's (friends with wine) that can provide you with a class set of corks.  Real corks not the new stuff they are plugging wine bottles with.  

Wine corks and cork flooring are made from the cork cambium of the cork oak (Quercus suber).  The cork oak is native to southwestern Europe and Northwestern Africa.  Portugal is the biggest supplier bringing in about half of the world's harvest of cork annually.  You can't just plug a bottle with any old tree bark.  Well you could, but I wouldn't suggest it.  After planting you have to wait 20 to 25 years to get the first cut at the bark of a Cork Oak.  Like most trees you don't just shave off the bark or risk girdling the tree.  Instead, it takes a skilled hand to peel off the thick bark but leave the phloem and the cambium to keep growing for another harvest in 10 years.

The value of the cork oak is unparalleled in the plant world.  There is no other substance in the plant world with the combination of physical properties contained in the cork oak.  Cork is packed with millions of cells per square inch filled with air giving it a density 0.193 grams per cubic centimeter, which is less than 1/4 the density of water.  Which is why it floats, a density greater than 1 will make solids sink in water.  These air packed cells are very resilient and will constantly exert pressure when squeezed providing a great seal to bottles without losing strength for more than 100 years.  The cells are arranged in a honeycomb like pattern that help give the cork some of its other physical properties.  Cork also contains a fatty acid known as suberin, which is very hydrophobic giving cork another benefit in sealing bottles by repelling water.  Suberin is also the substance commonly found in the roots of Mangroves which keeps saltwater out of its vascular system.  Cork will soak up some moisture, but compared to other forest products they will all come in second behind cork.  These thin layers of cells and phloem allow cork to be a great insulator of heat and sound giving it more uses in flooring and even some refrigeration set ups.  Cork is also hypoallergenic, meaning it resists molds, mildews, dust mites without the application of any additional chemicals.  It is also abrasion resistant which is great for the wine aficionados who don't like to have pieces of the bottle toppers in their evening drink.  Cork's home climate of the Mediterranean provides another great benefit to cork, fire resistance.  Not as critical in wine making but a great benefit to cork flooring.  


Physical properties lab:  

Make a friend in the science department and borrow their microscopes, one for every two or three students will do.  For this lab I would prefer dissecting, but compounds will work.  You'll just need to cut slices from each specimen to view with the compound microscope.  Break open the corks and compare their structure to that of other oak tree bark, pine bark, or other tree species in your area. 


Items needed:  

Bunsen burner, striker, microscope (dissecting), containers to hold water, water, food coloring, and a weight (brick will do)


Questions to answer:

What is the cell arrangement of the different types of bark?

Does each type have the same elasticity when compressed?

Can you isolate the cork layer from the other tree species?  Do they float as well as Cork Oak?

How does the fire resistance compare?

Weigh each piece before and after you place them in water.  Tint the water with food coloring.  Hold them underwater for 5 minutes each.  Do they all repel water equally?  Cut them open.  Can you see any absorption in the outer cells with your eye or under the microscope?

Try Styrofoam peanuts, rubber stoppers, wood, and similar items.  What are the benefits and drawbacks of each of these?


Good luck and have fun.


Brian Tomlinson