A note on screw and nail hardware joinery .
Topics are organized into the following groupings:
Best Uses for Flatsawn, Riftsawn, and Quartersawn Grain (to be added)
Hand Planing to Bring Out Grain and Figure (to be added)
Anticipating Seasonal Wood Movement (to be added)
Shooting Boards (to be added)
Spring Joints (to be added)
Mortise and Tenon (to be added)
Hand Cut Dovetails (to be added)
Breadboard Panel Ends (to be added)
Hot Hide Glue (to be added)
Birds mouth shelf supports, drawer slips, scratch beads - these are all examples of character-rich details lacking in modern mass-produced furniture.
Scratch Beading (to be added)
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An initial best practice would be to monitor regularly the relative humidity of the workshop. This can be accomplished most accurately with a gauge or meter. Relatively inexpensive ones can be purchased from an electronics store. It's valuable to know how the average relative humidy of the workshop compares to that expected in a home or office.
Wood is always reacting to surrounding conditions. As described in Working With Wood, a changing relative humidity can cause wood to expand or shrink across its grain. The challenge of making well fitting joints, drawers and doors is made more difficult if the wood is expanding or shrinking as it is being worked.
Newly obtained lumber should be given sufficient time to acclimate to the workshop before being worked. As a general rule, kiln dried lumber should be allowed to acclimatize for a minimum of two weeks when brought into a conditioned (heated and/or air conditioned) workshop from a warehouse or similar environment. Air dried lumber should be allowed to acclimatize for at least six months when brought into a conditioned environment. Rather than to follow general guidelines, however, a better practice is to measure with a meter the moisture content of new lumber and compare it to the moisture content of wood that has been in the workshop for several months. Allow the new lumber to continue acclimatizing until its moisture content is in line with wood that has been in the workshop for several months.
All surfaces of lumber need exposure to the air so that the moisture content of the wood can come into equilibrium with the surrounding relative humidity. To achieve this, the lumber is stacked and stickered . On a firm surface, 3/4" x 3/4" sticks are placed every 12 to 16 inches and the first board laid on top of them. Another set of sticks are placed on the board in line with the first set of "stickers" and the next board placed on them, and so on.
Once a piece of wood has been straightened, it may still be too thick for the intended use.
Saw it into two thinner pieces, and the internal stresses are no longer in equilibrium and the moisture content is not balanced. As a result, the two thinner pieces almost certainly won't be perfectly flat and straight. If a piece of wood is going to be "re-sawn" into two thinner pieces, that should be done as part of the initial cutting of the rough lumber so there is time for the two pieces to adjust and be flattened and straightened again.
Similarly, make the too thick piece thinner by removing all the excess wood from one side and the internal stresses and moisture content are again out of balance. To make a flat and straight piece of wood thinner, remove an equal amount of wood from both sides.
One might think that furniture kept year round in constant temperature would not suffer expansion and contraction. But the moisture content of wood changes not with the temperature of the surrounding air, but the relative humidity of the surrouding air.
Over the span of a few weeks in a stable environment, "dry" lumber reaches an equilibrium moisture content, based on the relative humidity of the surrounding air. Furniture will also reach an equilibrium content based on the relative humidity of the surrounding air, but may take longer. Furniture finishes are not waterproof, but most are more impervious to the movement of water molecules than raw wood. Therefore the moisture content of wood with a finish takes longer to reach equilibrium with the relative humidity of surrounding air. (Not all finishes are equally resistant to water molecule transmission - shellac, nitrocellulose lacquer, varnish, and acrylic are likely more resistant, boiled linseed oil, tung oil, danish oil, and wax are likely less resistant.)
The effect of finishes is not to prevent change in the moisture content of the wood, but to slow the change and possibly avoid extremes. If after several weeks of normal conditions, the relative humidity drops below normal for four weeks, then rises above normal for four weeks before returning to normal, the moisture content of unfinished wood in that environment will experience greater swings than the moisture content of furniture protected by a moisture impervious finish in the same environnment.
"The Moisture-Excludng Effectiveness of Finishes on Wood Surfaces" (1985) found some interesting results. Solvent based itrocellulose lacquer is not very effective at inhibiting moisture movement into and out of wood (page 10). Paste wax has no effect on moisture movement into and out of wood (page 18). In general, two coats of finish are more effective than one, and three coats are more effective than two (pages 10, 14). Moisture moves out of wood more slowly than it moves into wood. Thus it's possible that with equally long intervals of increased and decreased relative humidity levels in the surrounding air, the moisture content of wood may increase (page 20). Finishes don't lower the final mosture content of wood, at best they slow the rate of change (Page 29). The second most effective finish was molten paraffin wax applied by dipping; effectiveness was greatly reduced when applied by brushing (page 30). One conclusion of the study is that all surfaces of furniture wood should be coated with finish, not just the show surface; ideally, the edges of trapped panels are also coated with finish (page 30). The study results could be applied to end coatings when air drying lumber. The only interior non-pigmented aqueous or water borne finishes tested were an acrylic gloss varnish, an acrylic satin varnish, an alkyd varnish, and an acrylic gloss wood finish (page 32). An updated test with modern water borne non-pigmented finishes might be very interesting.
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