I was involved in research into preservation of bee boxes in the early 1980's.
I was very much influenced in later times by work carried out by Kalnins and
Detroy in the USA.
 
As has been stated on Bee-L before, pentachlorophenol (PCP) is definitely out
for boxes and, when used on exterior parts such as bottom cleats, gives no
real protection from termites (white ants).  I can vouch for this from
experience.
 
The copper chrome arsenate (CCA) is not suitable for boxes but is quite good
for the bottom cleats where there is no exposure of the wooden part to the
bees or honey.
 
bis-Tri-N-Butyl tin oxide (TBTO) does not have a good record in tropical or
sub-tropical areas as a preservative.
 
Copper naphthenate is the best we tested and the easiest used by home
treatments by the beekeeper.  To obtain the level of copper in the wood
required under laws in the State of Queensland in Australia, it was found that
a 1% solution of copper naphthenate would deliver the required level of
preservation if the timber was completely immersed in the solution for 8
hours with suitable spacing between the pieces of wood.  If you use a wrap and
soak method, then a solution of one and a quarter percent was necessary with
wood soaked for one hour then tightly wrapped in non porous plastic for a
minimum of four days.
 
If you use zinc naphthenate because you do not want the green colour on your
wood, then you have to double the strength because zinc naphthenate, when
compared to copper naphthenate, is not as effective a preservative as copper
unless the strength is increased.
 
New Zealand had used a product called Cee-Bee consisting of copper, chrome and
boron but I am not sure if that is currently being used.
 
The important aspect to remember in all timber preservation is that only the
sapwood is treatable.  The true wood (or heartwood) is not treatable.  The
other aspect to consider is what sort of risk of decay are your boxes exposed
to.  If you are in a tropical or sub-tropical area , if you do not treat your
boxes, they will only last a year or two.  If you are in a desert area with
low humidity, then your boxes will last a long time untreated.  Timber must be
over 20% moisture content before it will start to decay (rot) and then
temperature plays a big part in the decay process.  There are dry rots but
these are not common.  For instance, we do not have dry rot in Australia.  You
must have moisture and heat to start the decay process.
 
I am not sure how copper naphthenate is sold in other parts of the world but
in Australia the most common form used by beekeepers is the 5% copper.  There
are formulations called 40% copper naphthenate but these are only 4% copper.
About the strongest you can buy in Australia is 5% copper which would be 50%
copper naphthenate.
 
Mineral turpentine was the solution used to break down the copper naphthenate.
The green colour on the outside is not a good indication of the level of
retention of the copper in the wood.  This can only be done by chemical
analysis.
 
I hope this helps someone.
 
Trevor Weatherhead
AUSTRALIA