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An Introduction to the identification of wood from thin sections (with applications to fossil wood found in Quaternary deposits) **

By: R.J. Mott

Emeritus, Geological Survey of Canada
Department of Geography, University of Ottawa

Making sections & Slides

There will be a demonstration of the procedure for making thin sections in class, and you can try to make a few yourselves.

WARNING:

  • THIS TECHNIQUE USES XYLENE, PERMOUNT AND ETHYL ALCOHOL. PROPER PROCEDURES SHOULD BE USED WHEN HANDLING THESE PRODUCTS. CHECK WITH YOUR LOCAL PRODUCT SAFETY OFFICE.
  • THE MICROTOME BLADES ARE EXTREMELY SHARP AND CAUTION MUST BE USED.

Equipment:

  1. Microtome, sledge type (Model 860, American Optical)
  2. Mictorome Blades, disposable type (S35 type, Feather Co)
  3. Xylenes (Fisher X-5)
  4. Hematoxylin Stain Solution (Fisher SO-H-16)
  5. Ethyl Alcohol (95% and 100%)
  6. Petri dishes (5 are needed - stain, ETOH95%(a), ETOH 95%(b), ETOH 100%, Xylenes)
  7. Microscope slides, 22x40 coverslips, labels
  8. Permount (Fisher SP15B-100)
  9. Small brush, forceps

Procedure

1.Cut off a block of the wood of approximately 1 cm3.

Take a portion from the best part of the wood available. Try to avoid the branches, etc. When cutting the section from the disk, get as right an angle as possible. If you have a choice, use the outside of the section.

2. Boil the block of wood in water.

This softens the wood to make the thin sections easier to cut. Boil the wood until it sinks.

Fossil wood: keep it wet until you make the section; don't let it dry. Once fossil wood dries, it shrinks and the cells collapse. You usually don't need to boil fossil wood. Some woods can be soaked in KOH to soften.

3. Clamp the wood in the microtome.

The wood specimen should protrude only 1/2 cm.

4. Start cutting the sections.

  1. Start with the cross section. In the clamp, the annual rings should be horizontal. Throw out the first few sections. Keep the block moist to reduce curling. Keep the blade moist with ethyl alcohol (ETOH) and water mixture (½ -½). If fossil wood is moist and too soft, soak it in ETOH. Put a bead of water on the tree section and on the blade. Cut several sections and drop them in a dish of water. It helps to use the brush and stroke the section as you are cutting it to keep it from curling.
  2. Then cut the radial section. For this section, the annual rings are vertical in the clamp. This is the hardest section to do. You may want to put some sections on a slide to ensure that the rays are properly oriented. You can use the ball and socket to properly orient the block relative to the blade. Remember to keep the blade wet and a puddle of water on top of the wood. Use a small brush to pull off the wood and put it into water.
  3. Then cut the tangential sections.

5. Remove the sections from the water with tweezers.

Take a few and drop in stain. Let them sit for a while. Note: stain modern wood, but fossil doesn't need stain. Then put them back in the water (they should be blue) to remove some stain, put a drop of acid in the water.

6. Now run the sections through the washes.

  1. Put in 95% ethyl alcohol. (stir around a bit; leave for about 1 minute).
  2. Put in 95% ETOH bath #2.
  3. Put in 100% ETOH.
  4. Put in xylene (note: if any water is left in the sections, it will become cloudy).

7. Place the sections on a slide with some permount.

Place a drop of permount on the slide. Taking the section from the xylene, touch it on some paper, but don't draw off all the xylene. Orient the sections properly, place a drop of permount on top, and then the cover slip. Make sure to put the cover slip on before the xylene dries, or you will get bubbles. Place the slides, with permount and cover slip, on a hotplate (50 o C) and leave overnight. Add a small weight on the coverslip. This prevents the sections from curling and lifting the coverslip.

8. Scrape off excess permount, and clean with xylene. Label the slides.

 

**This page describes the method used to make thin sections of wood. It is based on my notes taken during a a short course on Wood Identification offered at the University of Ottawa, Department of Geography, June 1998. The methodology is that used by R. Mott, of the Geological Survey of Canada, and this page is taken from the short course. K. Gajewski.

 

 

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