Subject: Why would cooling an object like wood and metal increase its friction?

I did a science fair project on friction.  I build a slide ramp and slid 
down a block of wood (one inch by one inch), a block of wood wrappedin 
foil and a block of wood wrapped in cellophane.  I slid each object down 
the ramp five times at room temperature and again after they were in the 
freezer for three hours.  The angle required to make them slide was higher 
when they were frozen.  I hypothesized that because molecules move closer 
together when cooled then the surface imperfections would reduce which 
would reduce friction.  Any ideas I did not see this?

Re: Why would cooling an object like wood and metal increase its friction?