Special drill bits and tools are needed if you will be extracting cores from archaeological samples where the wood (usually hardwood) is very dry. Regular increment borers are insufficient and often unable to extract sound cores from such samples. Instead, dendroarchaeologists use a specially designed hollow drill bit driven by a heavy-duty industrial-style drill.
This is the company I use to create my custom-designed drill bits. The package comes as a three-piece kit that includes the 10" long drill bit, a polyethylene guide plate, and a sharp, curved extractor to dislodge the core from inside the wood beam. The cost for the 3-piece dry wood borer kit is $US 159.95. If you'd like to learn more about how to use of a dry wood borer, be sure to read my tutorial for properly and safely using the drill bits. The drill I prefer is a DeWalt DW245 7.8 amp 0.5 inch variable speed drill ($140.00) and you can order this from my online tree-ring supply store. I will say that I've used battery-powered (cordless) drills in the past also, but prefer to use corded drills for a continuous power supply and a lighter drill.
Extracting good samples from dry wood is a delicate job. A good tool is the prerequisite for every researcher investigating construction timber. RINNTECH offers dry wood borers with 8/16 mm diameter. They were developed by Thomas Bartholin and slightly improved by RINNTECH. RINNTECH dry wood borers can be run with commercial electric drills (600 W power). They are available in lengths of 150 mm, 250 mm and 350 mm. Application is possible in softwoods as well as in hardwoods. A maintenance set allows cleaning and sharpening of the borers.
The borers were developed in cooperation with the dendro laboratory of the Deutsches Archäologisches Institut Berlin (the German Archaeological Institute, Dr. Karl-Uwe Heußner DAI, Ref. Naturwiss, Im Dol 2-6, Haus IV,D-14195 Berlin) to be able to carry out samplings with minimal side effects. They are suitable for dendrochronological examinations especially at historical construction timber and are conceived for the use in solid dry wood. Fresh wood can be bored more or less well depending on its nature. Boring into wet wood causes problems with the chip removal. Moreover, the wood core can become wedged in the borer by the swollen condition.
Rob Wilson adds this information: "I used three dry archaeological corers in Germany which cost me about $US 20-30. They were home made. One can buy cutting bits (with teeth etc) at most home hardware stores. I used two sizes, 14 mm and 21 mm. The length of these bits varied from 2 cm for the wider to 3.5 cm for the thinner. Luckily I had a friend who worked in a machine shop, who welded the bit onto a tube of same diameter and thickness. This is the tricky bit, because the join must be flush and smooth (inside and out).
"The opposite end...i.e. the end that goes into the drill is also problematic. The Tucson system has a separate attachment that the corer twists into. Mine was a one piece system. Again a thick walled hollow piece of metal (but smaller diameter) was welded onto the end. The diameter of this can vary, but this depends on what diameter bits your drill can take. This piece must have a whole through it, because you will quite often want to push the wood sample out of the corer.
"A little refinement on the Tucson system was an idea that I picked up from the archaeological lab in Sheffield. At periodic intervals down the bit, holes should be drilled through the walls of the corer (ca. 5 mm). Mine were drilled in a pseudo spiral pattern of about 7 cm distance apart. Because one is coring dry wood, there is a lot of dust that can clog the corer and sample when drilling. A lot of dust will come out of these holes. Be sure to smooth the burr created when making theses holes.
"The problem still remains on how to get the sample from the beam. Quite often once you have drilled your 30 cm into the beam, the sample will still be attached to the beam (though it can sometimes come out in the corer). You need an extractor. Bicycle spokes are good for this. You want to bend one into a L shape with the longest end being the same length of your corer. At the end of this, with some sharp pliers, you want to 'pinch' a sharp point at right angles. When taking the sample out, there will be a gap around the sample and the beam. Enter the extractor along this gap as far into the beam as possible. When you reach the end, twist the extractor so the sharp point will attach itself to the bottom end of the sample and pull. The sample should come right out in one piece."
Geoff Downes adds: "CSIRO developed a corer for a powered drill bit which should do the job. It extracts a 12 mm core and leaves a 22 mm hole. This is a standard dowel size for plugging. It comes in two lengths, 300 mm and 500 mm. I have used the corer myself and it does a good job. The only drawback is the need to return it to the suppliers for re-sharpening. They are yet to supply a sharpening jig for others to do it." Contact CSIRO at: CSIRO Forestry and Forest Products, GPO BOX 252-12, Hobart, 7001, Tasmania, AUSTRALIA.
Olafur Eggertsson writes: "You should contact Thomas Bartholin in Copenhagen - he has the best corers, with prices from c. 4000.- DKR."