Hi all, I'm looking for some advice about slightly increasing the moisture content of timber rapidly. I make circular tables that I edge with a steel ring. I'm really careful to dry the timber in my kiln prior to joining and cutting the boards to create the top, and I then roll a steel ring that fits very tightly. The timber I use is Norway spruce and I try to get a <7% surface/12% core reading with my moisture meter so that when the tables eventually go into a home they'll take on a little moisture over time but not shrink (resulting in a gap between the edging and the timber). This has worked so far without any issues, but I have one table top that's undergone further shrinkage after fitting the edgeband because I had to keep it in the kiln overnight to cure a finish. I didn't switch off the dehumidifier.
I am currently attempting to rectify the problem by putting the top back into the kiln and injecting steam to increase the RH, but I don't know whether this will work in the time I have left prior to meeting the deadline for shipping. The worst case scenario is that I roll a new steel ring to replace the current one, but I'm looking for something that's a fix for future instances of this problem.
These tables sell very well and I don't want to change the design or stop making them. Can anyone offer me some advice?
With a moisture gradient between the core of the wood, and the surface, it's going to move. Steel frames and wood don't play well together. Wood is always going to expand and contract. No amount of kiln drying is going to stabilize it for no shrinkage in the future! It swells one time, it may crush some cells to stay in the ring, but then shrink away when the humidity drops. Especially considering those wide flat sawn planks. Quarter sawn would be a must for me, but if you don't want to see any gaps, you should run a rabbet around the table and have the wood set over a ring. I'd say you are still going to get some issues. Can't fool Mother Nature.
Rich is correct except for the part that "no amount of kiln drying will stabilize it for no shrinkage in the future". There is a way....
Get all your boards to a good 7-8% all thru the lumber, then build the table and finish it well. Then rush it to a museum with state of the art HVAC set to make stable humidity and temperatures. This preserves the art work that is often fragile and at risk with temperature and humidity variants.
You can see the old style chart recorders in the better museums as a means of protection and insurance. Newer ones are digital recorders, but they serve the same purpose. This what helps keep those 200 yr old pieces of furniture looking good. It can work for your tables also.
If I read your post correctly, you have a problem with one table because you made a variation in your production process to only this one table.
If you have had no issues in the past with however many tables you have built, perhaps the problem is confined to only this table and the single episodic variation from your standard production process.
In the States we say "If it isn't broke, don't fix it." You may come out ahead, from both a profit and stress standpoint, by rebuilding the errant table using your standard process and moving on.
I would only concern myself with this shrinkage issue if it proves to be replicated on tables that you have built using your standard process.
After looking at the table top I read your name and smiled. Anyway, I agree with the first couple of responses. No matter what you do the boards will expand/contract with changes in RH. There is no way to stop that. The way I would deal with the issue is make the steel hoop in two pieces, with one overlapping the other. Orient the ends of the hoops at the centerline of the table top. Attach each hoop to the boards but not to each other or, if you do, make sure that connection can slide. Now when the wood expands/contracts the hoop halves will move with it.
Thank you very much for the responses, guys! Quite right about the change in the production process for this top, I shouldn't have put it in the kiln with the ring on it - bad idea.
I have considered putting a flat ring into a rabbet to hide gapping and help support the edge ring, but rolling one accurately with my "hand mangle" would be very challenging indeed, and it would add cost to the customer. I have also considered making the ring in two or more parts, but because all boards are able to contract at the ends then isolated gaps are still possible IYSWIM.
I've looked at PEG but I don't have any of the required equipment (vats etc) and can't dispose of the waste easily/cheaply, and in any case it ought to be unnecessary given that I've had no other issues (YET!) apart from the very first table I built this way (which has just come back to me for the second time for a ring refit... I learned a lot from this one).
I've decided to halt production for a few weeks so that the timber that's in the kiln right now is dried to a point where it has little or no gradient, and I think this will produce the safest material to work with. Fingers crossed.
Does anyone have a suggestion for the least permeable finish available (in the UK if possible)? I'm thinking that if I can slow moisture exchange as much as it possible (by coating all surfaces including end grain with it) I can mitigate the risk of movement even further...
"After looking at the table top I read your name and smiled" - haha, well spotted ;-)
Your wood is going to move with changes in RH no matter how much you dry it. The only way to stop wood from moving is to keep it at constant RH. Since that's not practical in any setting other than a museum you need to adapt the ring to accommodate the wood. The other way around will never work with that design. Finish only slows the exchange of moisture, it won't stop it.
You need to make a design change in order to solve the problem.
Yep, no matter what you do with the kiln or finish, the rings are going to be loose with flat sawn stock during heating season. Always, always, always! People crossing the prairies in Conestoga wagons were always glad to ride through water once and a while. The rims would tighten back up on the wheels.
I agree with the other post regarding the inevitability of wood movement. To answer your question regarding a finish, de-waxed shellac provides the best barrier to water vapor. You can then finish with whatever finish you like as most anything will stick to it.
Yes, I had the same question in my mind too. Why this particular table alone? If this problem had not occurred before, then your procedures for this isolated incident could be overlooked. To save time, I would simply use a new steel ring to fit into the shrunk diameter of the wooden table. I have a similar storage cabinet made of this wood and it still shrinks occasionally but not so obviously. So, I guess it is just inevitable nature doing its course.
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