Having that natural edge: using timber slabs

Above: A pair of tables featuring camphor laurel slab section tops by by Nicholas Arthur with steel work by Groove Fab. Wood salvaged and machined by Alastair Boell. Photo: Tegan Steele

Words: Linda Nathan
Photos: Alastair Boell

Lined up, standing end-on to best display their glorious figure and colour, timber slabs are one of the more eye-catching features at wood shows and select timberyards. Although they comprise a small part of the timber sold in this country, they present a unique opportunity for wood users and wood lovers alike.

Slabs of cedar, cypress, camphor laurel, elm, sequoia, redwood, blackwood, blue gum and red gum amongst others are likely to come from a diminishing resource of larger trees, and may display astonishing figure and feature. They are sourced from all over Australia, mostly the product of rural, forest and urban recovery. They can be milled from freshly felled timber however some argue that slabs from salvaged logs are better.

Cedar salvaged from Ivanhoe Grammar, Victoria milled in through and through slab sections by Alastair Boell

A slab is technically a piece of timber which retains part of its natural edge; once these are removed the wood is classified as sawn timber. Literally a slice of a tree, a slab is a warts’n’all kind of thing which may or may not contain sapwood, heartwood, splits, cracks, borer holes, gum vein and other natural feature. Different species and different trees will exhibit different feature. Knowing this explains why a slab of timber can’t be evaluated or priced the same way as graded, sawn timber. Not all trees may be slabbed bark-to-bark; species such as silky oak, for example, are vulnerable post-harvest to infestation by pinhole borer.

Buying slabs

Just because a slab may contain various features or defects, doesn’t mean the would-be buyer shouldn’t be discriminating. Slabs must be seasoned before use, that is, either air- or kiln-dried to the accepted 12–15% equilibrium moisture content (EMC). Any lyctus susceptible sapwood should be immunised to prevent ongoing degrade.

There is some debate as to which is better for slabs, air- or kiln-drying. One or the other, or a combination of both may be favoured. As with all things relating to timber, there are many variables which account for differing opinions. When in doubt, consult your timber supplier.

Eye-catching potential from bookmatched slabs

Whilst the visual appeal of feature may be in the eye of the beholder, the structural integrity of the wood in relation to the feature it contains must be objectively considered. A good slab is preferably free of heartwood and quartersawn timber is more stable as any interlocking grain or figure will be better highlighted.

Slabs which are not totally sound will be resawn or ‘optimised’ to produce standard sections. Wide boards have a tendency to cup and need appropriate handling. Other features to watch out for are excessive wane, that is the slabs or slices that are close to the ends of the log. These boards will have a steep angled edge and will be more likely to cup, curving always towards the outside of the ‘tree’. Slabs from the centre are therefore more stable for use in furniture. It may also be possible to obtain sets of boards which can be bookmatched or otherwise displayed to effect.

Most slabs are sold at a cubic metre rate but some are priced at ‘piecemeal’ rates, where their particular feature or rarity is uniquely valued. Green timber will cost less than seasoned, however it’s only the latter which should be used for the benchtops, bartops, tabletops, counters and cabinets which slabs are commonly used for. The extra cost of properly seasoned timber reflects the overheads of the timber producer who has spent time and money selecting, milling, storing and drying their product.

Advantages

Visually it’s not hard to see the advantages of this resource. Large wide boards best display grain, colour and feature and this makes them a very marketable commodity. The rarity of some of these pieces alone can add considerably to the value of a piece. Utilising the wane or bark edge can lend a sculptural, earthy or rustic effect which can also be a sale point. Wide boards are much rarer now than they used to be, another feature which adds value to a work and which may remove some of the work of laminating boards.

Disadvantages

The size and width of the slabs can make them harder to manhandle and also mean more time spent machining them. You mightn’t have to join boards but your slabs may not fit through your planer/thicknesser. Unless you have a very wide thicknesser or sander you may have to have your slab/s machined at a workshop, factory or timberyard which offers this kind of service. If you don’t want to pay someone else, you’ll have to flatten your slabs with an electric power planer and sanders. Supply of slabbed timbers can be variable. Whilst some merchants offer seasoned slabs ‘off-the-rack’, buyers need to know that available species and sizes will vary.

Joinery with slabs

Your slabs may show 12–15% moisture content but they will continue to move. The following may help to keep your boards flat:
1. Select pieces closer to the centre of the log; these will be more stable.
2. More heavily figured pieces will be more stable than those which are straighter grained.
3. Ensure slabs are correctly seasoned.
4. Use appropriate joining and fastening systems. It may be appropriate to design your job using a system of through dovetails and/or cleats. Screwing tabletops to frames invites disaster no matter whether using slabs or joined boards.

Designing with slabs

Beautiful natural-edged sections of wood are not enough on their own to create well designed furniture. Marrying odd shapes and sections can give rise to what some have called ‘Fred Flintstone’-type furniture… The same principles of good design with regards to aesthetics and function apply to cabinets and furniture created with slab timbers and it’s best not to err on the side of overkill.

A great part of the appeal of using natural-edged timber sections lies in the fact that you can still see evidence of the source of the wood, the tree itself. Incorporated into domestic and commercial situations they can serve as reminders of the infinite variety of naturally occurring forms.

Photos supplied by Alastair Boell, who in addition to teaching woodwork also mills and sells salvaged timber, see www.mgfw.com.au