How to make a kiln for drying hand cut veneers

Words and photos: Neil Erasmus

One of the most compelling reasons a maker may decide to use veneers is the great freedom they afford in arranging grain direction and species of timber. Solid timber has its limitations. It's subject to shrinkage, warping, cracking, twisting and bowing. Some timbers are exceptionally beautiful but rarely used in solid form because of their inherent instability and resulting degrade upon drying.

I find commercially sliced veneers too thin and the surface texture a little coarse. The integrity of of sawn veneers remains the same as solid wood, and they tend to appear more natural than sliced veneers. Once the veneers have been cut they are carefully stickered out and dried in a purpose-made kiln or simply hung on a line, then sanded on a drum sander that takes them down to a uniform 1.0–1.5mm.

I prefer to saw veneers while the wood is still wet. It cuts well while it is still at a stage before end checking can occur. I hang the veneers to dry in my workshop – this brings the moisture content down to EMC (equilibrium moisture content.) This is fine for commercial veneers or sawn veneers that are taken down to 0.75mm thick or less. However after considerable trial and a little error, I now believe that thicker veneers remain more stable on the substrate once they’ve been dried to well below EMC. Depending on the species of timber, I aim for a MC of 4 to 8% (in Perth).

Naturally, this is impossible without subjecting such material to a humidity controlled, dry environment for several days. To achieve this, I devised, some years ago, a very simple but effective kiln, that I could easily afford and build myself.

The kiln

Necessity being the mother of invention, I designed my kiln when confronted with the need to dry 50sqm of sawn veneers, the longest of these being 2.2 metres. I guessed that the kiln I had in mind would dry 2–3mm thick rough sawn stock in two to three days, and seeing I had a little time to spare, I was happy to do it in several batches. I therefore didn’t need to make a huge kiln which would also have meant increased heating and de-humidification capacities. In addition to this, as I wasn’t sure what the results would be, I didn’t wish to invest too much time or money.

The basic requirements were a heat source, a system to circulate the warm air over both faces of each veneer, a dehumidifier and a well sealed enclosure. I started with the latter. I finally felt vindicated for insisting that no one throw away a number of 25mm particleboard cover sheets I had collected. I cut these up to size and nailed together a box measuring 2400 x 1200 x 1200mm high. This box was then fitted with a pair of 1200 x 1200mm doors which, when opened, exposed the entire front area of the kiln. While this kiln is a little crude, it does its job by forming the backbone of a drying kiln I regularly use.

Essentially, what is needed is a fairly robust structure, with full access from the front to allow easy packing and doors that close and seal firmly. In addition to this it is wise to thermally insulate the interior to help keep the power bill down.

At first, I fitted a pair of dual 1200/2400-watt fan heaters. These worked, well—too well, as they were not thermostatically controlled and resulted in melting some of the polystyrene insulation! I felt that they were a definite fire hazard and opted for an oil-filled, thermostatically controlled heater instead. I strongly recommend the latter, as there is no risk whatsoever of igniting a fire.

As the heat build-up inside the kiln can easily exceed 50°C it is important that the heat source automatically controls the temperature to the desired setting. The temperature, if too high, can destroy the sensitive workings of the dehumidifier that also resides inside the kiln.

Inside the kiln, showing the dehumidifier, heater, secondary boxes, fans, thermometer, sensor and insulation.

The mobile oil heater I use is a 2400 watt unit fitted with a dial to adjust the temperature. These heaters are surprisingly efficient, quickly warming the air in the kiln, which I maintain at a constant 34-36°C. To fit the heater, I cut a neat hole in the end of the kiln, sliding the radiator through and leaving the control panel exposed outside. I then carefully sealed up any gaps by stuffing bits of pink batt insulation into them.

An inexpensive, digital thermometer was then fitted by hanging its sensor probe through a small hole in the top of the kiln and placing its display screen and controls near the heater’s control panel.

One word of warning here: These thermometers are designed for cars or homes and feature a remote sensor probe attached to a pair of wires. The actual device itself also contains a built in sensor and a switch that the user places in internal or external mode. The remote probe that hangs inside the kiln is active in external use.

Secondary boxes

A long, narrow, secondary box fitted with four fans in the back sits on the floor of the main box. It is a smaller version of the kiln in that it is also a 5-sided box with its long front part completely open. The sawn veneers, which are sandwiched between layers of corrugated cardboard, are carefully packed inside this box and clamped down to keep them flat as they dry.

The purpose of this box is to concentrate airflow within a smaller volume, while still keeping the entire unit sealed. I also have three smaller boxes, the largest of which is about half the length of the long one, and the smallest is just big enough to dry the specialty timber veneers I use for document boxes and inlays. These smaller, secondary boxes, which are high enough to fit snugly inside the long one, have holes in the back, which coincide with the fans fitted to the ‘mother’ box.

The dehumidifier

In a sealed kiln such as the one described here, the humidity level would rise considerably as the warmed air constantly circulates and wicks up moisture from the veneers. Without some means of removing the moisture from the air, it would be impossible to dry the veneers properly.

The dehumidifier, which is rated at 15 litres per day, was an ‘off-the-shelf’ unit I purchased together with the heater from a household appliance store. It featured an adjustable, automatic humidistat and a hose that carries the condensed water, by gravity, into a bucket which is placed outside the kiln. This compact little piece of equipment sits just behind the veneer storage box on the floor of the kiln, and is dialed to ‘minimum humidity level’.

I must admit I was a little surprised when the machine actually lived up to its manufacturer’s claim by producing 15 litres of condensate in a single day! At the time, I had the storage box loaded to capacity with green veneers.

Packing the kiln

The rationale behind the drying of any timber is to allow airflow over both faces of each plank or veneer while keeping it flat. With thicker material, stickers placed every 400mm or so work fine, but thin material needs to be supported almost everywhere. I use corrugated cardboard that I buy in wide rolls and cut into several shorter ones to suit the width of the material I wish to dry. I simply cut the roll by hand, using a hand held saw.

Make sure that the pile of veneers are placed in their consecutive order, then carefully pack a suitably sized air circulation box, starting with a length of corrugated cardboard on the bottom. Please note that the corrugations need to run from front to back or at 90° to the veneers, otherwise they will impede airflow. Two lengths of cardboard need to be placed in between veneers, with the corrugations always facing the wood.

I am still regularly surprised at how well this system works, having experienced precious few mishaps. I have never had the problems (such as case hardening or surface checking) which can occur when drying thicker stock. For this reason you don’t need to re-humidify the kiln at any stage. Moisture within each veneer doesn’t have far to migrate and its journey to the surface seems to take place at the same rate as it evaporates. The surface therefore doesn’t really have a chance to dry out or ‘case harden’.

Once the stack of veneers has been packed, they need to be weighed down or clamped. I clamp them very firmly down by placing a piece of MDF, cut to the same size as the veneers, onto the stack and wedging several pieces of scrap in between the top of the stack and the underside of the box. Don’t ignore this step, otherwise you may well end up with buckled and twisted stock.

The next stage is to fill in any area of the box where airflow is not required. In other words, only leave the front of the stack of veneers and cardboard exposed to the front, covering up all the rest. Once the fans are running, the airflow will pass through areas of least resistance, and once these have been blocked off, it is then forced through the corrugations in the cardboard and thus over both faces of each veneer. Test this by placing a piece of paper over the front edge of the stack, if it is sucked up against it you know you’re in business. If it doesn’t; well, start again!

Switch on the fans, humidifier and heater, then monitor the temperature to make sure that it doesn’t exceed 35°C. Check the moisture content after about 24 hrs by removing several veneers from the top of the pile. I use a non-intrusive moisture meter.

Drying time may vary, depending on species, number of veneers, thickness and initial moisture content. As a rough guide: ten square meters of jarrah veneer cut to 2mm thick and measuring 30%mc. would take approximately 48 hrs to dry to 6%mc.

Sanding

Once I’m satisfied that all the veneers are properly dry and flat, they’re ready to be sanded to a uniform thickness. I try to aim for 1–1.25mm. I use a dual drum sander for this operation. This was comparatively inexpensive and appears a little light duty but performs surprisingly well. The advantage of two drums is that two grades of abrasive can be fitted, the coarser one cutting first and the rear drum adjusted to take a finer cut for a smooth finish.

The feed belt is fitted with a speed control and is used to regulate the way you wish to abrade. Some timbers have a very low burning temperature, and will burn very quickly if too much friction is applied. In such cases, adjust the depth of cut to remove less material, and speed up the feed belt.

One drawback when sanding thin stock is that the feed belt gets too close to the sanding drums for comfort, touching occasionally. Because the feed belt is also made of abrasive cloth, this has the effect of wearing out the grit on both drums and belt. I avoid this problem by using a piece of 9 or 12mm MDF as a base for each veneer as it is passed through the machine. To provide a stop to prevent the veneer from slipping out, I simply cut a groove across the face at one end, and slip a piece of 3mm MDF into it (see diagram below).

Two people perform this operation more easily, one feeding in and the other tailing out, as each veneer needs to be carefully held down to avoid buckling. Be careful with this operation, the last thing you want at this stage is to destroy your precious hard work. Sometimes I hold a piece of scrap wood over the veneer to keep it flat and up against the stop as it passes through the machine. If it buckles up and slips over the stop, stand clear, quickly wind down the feed platen and pull it out.

Abridged from Australian Wood Review magazine, issues 31 and 32, June and September 2001

Neil Erasmus @neilerasmus designs and makes furniture in Perth, WA.