What 8 to 10 percent actually means
Timber is hygroscopic. It gives moisture to the air and takes it back until the two are in equilibrium, and that equilibrium point is called the equilibrium moisture content. In a dining room held at 20 to 25 degrees Celsius and 45 to 55 percent relative humidity, most of the hardwoods used in restaurant furniture sit at 8 to 10 percent. A kiln target of 8 to 10 percent is simply the factory putting the wood at the moisture it will live at for the next fifteen years, before it cuts the joints.
The number matters because wood moves. Across the grain, a hardwood moves roughly 0.15 percent of its width for every 1 percent change in moisture content. On a 900 mm wide top for one of our wood restaurant tables, a two point swing in moisture moves the panel 2.7 mm. That movement does not disappear. It either goes into a designed allowance, or it goes into a cup, a split or an open glue line. Long grain moves about a twentieth as much, which is why a leg stays straight while the seat rail above it shrinks.
Drying below 6 percent is a different mistake. The wood goes into tension, the cells collapse slightly, and joints that were tight in the yard open in the first dry week of the heating season. It also costs 8 to 12 percent more in kiln time for a number that no room will ever hold. Drying to 12 or 14 percent, which is the normal export practice for construction lumber, is cheaper still and produces furniture that arrives wet and dries in service. Both directions fail; 8 to 10 is the window that does not.
The gap that travels inside the container
A factory in Foshan in July runs at 30 degrees and 80 percent relative humidity, which is an equilibrium of about 15 percent. Timber comes out of the kiln at 10, gets machined in that air, and the outer two millimetres pull back 1.5 to 2 points within about 48 hours of leaving the stack. That is why the number that counts is the core number, measured at depth, not the reading taken off a planed face.
Then the container. Twenty-eight to thirty-five days at sea, internal air temperatures that reach 55 to 60 degrees when the box sits in the sun on a dock, and a slow equalisation towards whatever the cargo air holds. Steel containers sweat. The timber that left at 10 percent often arrives between 10 and 12, and it arrives with a dry skin, because the same voyage that wets the core dries the surface.
The room at the other end is the hard part. A restaurant held at 21 degrees with forced air heating in January sits at 20 to 25 percent relative humidity, which is an equilibrium of 5 percent. Timber that left at 10 percent now has to lose 5 points in service. A 900 mm top shrinks 6.75 mm across the grain. If that top is screwed down tight at both ends to a steel frame, the 6.75 mm has nowhere to go and the panel cups by 1 to 2 mm in the middle, or splits 3 to 4 mm from the end. In a humid coastal site the same shipment gains moisture instead and the joints go tight, then the panels swell and the drawers and doors bind.
Why the failure takes three to six months
Nothing looks wrong at handover. Shrinkage in the first two weeks is mostly the outer fibres, which is 0.2 to 0.5 mm — invisible on a finished joint. The visible failure needs the whole section to dry, and a 45 mm leg dries through at roughly 1 mm of depth per week in a dry room. That puts the core at the room's equilibrium somewhere between week six and week eighteen, depending on thickness and airflow.
The failure order is predictable. First comes end checking: hairline splits of 0.5 to 2 mm on the end grain of legs, usually on the side that faces the air handling unit. Then the joints. On wood restaurant seating, the seat rail to leg joint is the first to move, because the rail is short grain in one direction and the leg is long grain in the other, so the two shrink against each other and the glue line creeps 0.1 to 0.3 mm. A mortise and tenon that went together as a press fit becomes a 0.5 mm gap you can feel with a fingernail, and a chair that was solid in month two rocks on a flat floor by month seven.
Then the finish, which is the part the guest sees. A two-pack polyurethane film will bridge a 0.2 mm joint movement and crack at 0.5 mm. The crack is where the cleaning water gets in, and the water accelerates everything else. By the time a manager reports it, the frame has been moving for four months and the cause — the moisture at packing — is two continents away and nine months old.
Drying that actually reaches the core
The schedule that works is slow and boring. Air dry under cover for 30 to 45 days down to 18 to 20 percent, which costs nothing but yard space, then kiln 10 to 16 days. The kiln starts at 60 degrees dry bulb and 55 wet bulb, holds a wet bulb depression of 5 to 8 degrees for the first third so the surface does not case harden, then opens the depression to 20 degrees, finishes at 70 over 62, equalises until every sample board reads within 1 point, and conditions at 75 degrees for 8 to 12 hours to relieve the case hardening that the fast part of the schedule built in.
Two details decide whether that schedule holds. Stickers every 300 to 400 mm at 20 mm thick, because airflow between boards is the whole mechanism and a 500 mm gap leaves a wet stripe that no meter will find once the board is planed. And 72 hours back in the sticker stack after machining, before assembly, so the freshly exposed core can equalise with the air instead of being sealed into a joint at a different moisture from the surface around it.
The cost of all this is 45 to 60 days of yard time and 3 to 5 percent on the piece price. The shortcut is a 6 day kiln run at a 25 degree depression: the boards read 9 percent on the outside, the core is at 16, and the meter agrees with the paperwork. As a restaurant furniture manufacturer working with kiln-dried Southeast Asian hardwood and oak, we would rather quote the extra 4 percent than carry a container of chairs into its second winter.
Ten minutes on the loading dock
A pin-type meter, driven 8 to 10 mm into an end grain face or a concealed surface, is enough to catch a bad shipment. Sample 20 to 25 pieces at random from different cartons, not the top layer of the first carton. Do not read the finished face: after six weeks at sea the outer 2 mm is drier than the core by 2 to 3 points, so a face reading flatters the shipment by exactly the amount you are trying to detect.
Correct the reading before you judge it. Most meters are calibrated at 20 degrees Celsius; at 5 degrees in an unheated warehouse the same wood reads 1 to 2 points low, so add 2 and re-read. Species correction is a second adjustment, 1 to 2 points for dense tropical hardwood against the meter's default. Write both numbers on the inspection sheet, because the difference between 10.4 and 12.4 is the difference between accepting and conditioning.
The acceptance line we work to: 8 to 10.5 percent for a temperate destination, 10 to 12 for humid coastal. Above 12, do not install — put the pieces in the room, unpacked, for two weeks with the HVAC running, and re-measure. Below 7, expect joint gaps in the first dry month even though the shipment passed. Once per shipment, cut a 50 mm offcut, weigh it, dry it at 103 degrees for 24 hours and weigh it again; the loss over dry weight is the true moisture content, and it tells you whether the meter or the supplier is the one that is wrong. That oven-dry check is the backbone of any serious restaurant furniture quality control routine, and it costs one destroyed sample.
What goes into the purchase order
Four lines in the specification do most of the work. Moisture content at packing, 8 to 10 percent, measured by oven-dry or by a calibrated pin meter at 10 mm depth. The destination equilibrium, stated as a number, not as a city. The kiln record, with the schedule and the equalising hold, provided with the shipping documents. And a line on the packing itself, because a wet carton or a wet pallet puts 1 to 2 points back into the furniture in transit; 20 mm foam against the frame and a desiccant bag per carton is cheaper than the claim.
Add one operational instruction for the site: unpack 72 hours before installation and let the furniture sit in the finished room with the air conditioning running. A chair that goes from a 12 percent container into a 5 percent room overnight will do in three weeks what it would otherwise do in three months, and the failure will be blamed on the factory. Written into the restaurant furniture specification, this is a delivery instruction. Left out, it is an argument. The risk multiplies when a room is ordered as solid wood restaurant table and chair sets plus loose chairs, because a set is cut from the same boards and every piece in it will move on the same day, in the same direction, at the same joint.
None of this is exotic. It is the difference between buying a photograph of a chair and buying a chair that holds its joints through a heating season, and it is the first thing we check when a buyer sends a claim from a previous supplier. If the room is a dry winter climate, specify 7 to 9 percent and accept the kiln time. If it is humid coastal, 10 to 12 is correct and a drier number will only make the panels swell. The full range, with species and section sizes, is on restaurant furniture products; the drying is the part you cannot see in any of the photographs, which is exactly why it is worth a clause.