|MadSci Network: Engineering|
Thanks for your question, Sheri. It makes sense to think that if you put combine two substances, both of which have strength, that the combined structure will be stronger than either of the components. This is most likely the case (although there are a number of cases where the strength of the system could be reduced – I won’t discuss any of those in this answer) but the difficult part of the experiment lies in the ability to measure the strength very precisely because the change in strength could be very small.
First I will explain what gives wood its strength. Then I will suggest a potential experiment that might show a difference in the strength of wood, if you have sensitive enough testing equipment. Finally, I will discuss why paint is used on wood.
The strength of wood can vary greatly and depends on a number of things since wood is a composite structure, but I will explain generally what gives the wood strength.
The strength comes mainly from two components of the wood, the fibers and the lignin. The fibers are like little rods that line up in the direction the tree grows and carry much of the load of the tree. These fibers are held together with the lignin, which is like glue that holds the fibers together.
The reason the wood was not stronger when you painted it is because the strength of the wood is much greater than the strength of the paint so you did not notice an increase in the strength.
One way you may have noticed a difference in the strength is if you would have used a very thin piece of wood and painted it with a thick layer of paint. In this situation, the strength of the wood might not overwhelm the strength of the paint and adding the paint could increase the overall strength of the system.
The reason paint is added to wood, other than for aesthetic purposes, is to keep water from penetrating the wood. When the water penetrates the wood, two things happen. First, the water reduces the strength of the wood. Wood becomes weaker as the moisture content in the wood increases, for example the rupture strength of wood is decreased 20% when the moisture content is increased from 0% to 5% (Panshin and deZeeuw, Textbook of Wood Technology, McGraw-Hill, 1980, pg. 227). Having the water in the wood also increases the chance that the wood will rot, which obviously will cause the strength to decrease.
Try the links in the MadSci Library for more information on Engineering.