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Sprung joints, moisture gradient, and finish

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Sprung joints, moisture gradient, and finish

#1

Sprung joints, moisture gradient, and finish

Denis Ch�nard, Orl�ans, Ont.

>Preamble:

My wife bought some el cheapo wooden cutting boards a month or so ago, from Ikea or somewhere else (laminated from 1 1/2" wide strips), I did feel a bit insulted to say the least...

Now, my dear wife doesn't pay much attention to wood's properties, to my great dismay. She can dunk one of those cutting boards in a sink full of hot water for half an hour or so before finally cleaning and rinsing it. Of course you know the result, after a few drown/dry cycles the glue joints open up at the boards' ends as a result of compression set shrinkage. With that kind of treatment, I don't think that sprung joints would save the day, nor would one piece boards...

Heart of the matter:

There was a discussion recently about sprung joints, debating whether it was worth doing them or not. For discussion purposes, let's suppose we're making a 30" wide by 60" long table out of four boards.

It seems to me that, in a temperature and moisture controlled environment, sprung joints are unnecessary. In real life it's another ball game though, as moisture changes over the year (in most locales).

Now, if one built the aforementioned table in the fall, sprung joints sound like a good idea, as the table's ends will lose moisture faster than the table top's middle, and a sprung joint would allow the table ends to lose moisture faster without ending up in splits at the ends.

However, if one builds the same table in the spring, then it seems that sprung joints would be a bad idea, as the ends will want to expand faster than the middle, potentially resulting in splits or failed glue lines in the middle of the table.

The potential for joints to split thus seems, all other things being equal, to depend on the "moisture gradient" of the wood species used. If a species allows moisture to flow quickly from the boards' ends to the middle, or the ends don't absorb moisture quickly, then would be little tension to cause splits and/or failed glue lines.

I don't know of any source for this "moisture gradient" factor by species, anyone knows of one?

Also, the finish used on our sample table will have an impact on the potential for splits. Shellac is renowned as having the best moisture barrier of all finishes. Anything else good in that respect?

Comments?

DC

A corollary to this is that breadboard ends should help diminish the potential for table top failure, as they cover the table top boards' ends, therefore reducing the endgrain moisture exchange rate.

Re: Sprung joints, moisture gradient, and finish

#2

where to look for guidance....

Bill Tindal, E.TN

>When deciding on how to make something I look at what someone did on an antique that has survived 200 years and/or what a furniture factory does that does not want a piece returned for a refund no matter where it is shipped to. In the case of glued up panels I take my guidance from the factories. They don't use sprung joints, they do use yellow glue, the lumber is at 7% moisture and the pieces are finished, often with lacquer. I have never seen a joint so made fail under normal living conditions. I use KD lumber at less then 8% MC, I don't spring joints, I do use yellow glue and sometimes I finish with lacquer. I have only had 1 joint ever fail and it was when I glued some cherry with hide glue and apparently the cherry was at too high moisture content. The cherry shrank and the joint failed at the glue line with no wood failure. This was the end of my use of hide glue.

If you make good joints and use dry lumber and yellow glue and finish your pieces, your joints will never fail under reasonable conditions. No doubt others will advocate more historical procedures to successfully accomplish the same goal. You can decide if your objective is to successfully make something or to reproduce a historical technique to make something. My point is not to criticize historical techniques for panel construction but to emphasize that it can be straightforward to make glued panels that last.

And be thankful that you didn't make the cutting board as it would have been painful seeing it soaked.

Re: Sprung joints, moisture gradient, and finish

#3

Cutting boards and sprung joints

Paul in NJ

>My wife runs our cutting boards through the dishwasher after every use. The cutting boards are one piece quarter sawn maple, mahogany, or white cedar, whatever was handy. They won't win any contests (but maybe 1st prize in an art show) but have been useful tools in the kitchen for the last twenty years.

I don't use sprung joints and never had panels split at the glue line. I have used both commercial kiln dried and my own air dried wood. My home is heated with wood so it is very dry in the winter and only our bedroom is air-conditioned. I can see definite wood movement from the extremes of the seasons. I wonder how easily you can squeeze those joints together on 7-1/2" wide boards and how much stress will that put on a glue line when you release the clamps?

Paul Dzioba

Re: Sprung joints, moisture gradient, and finish

#4

Re: Cutting boards and sprung joints

Sgian Dubh

>I think Paul it's possible that some people might have a slight misunderstanding when it comes to sprung joints. It's my experience that a slight spring is better than the reverse, a gap at either end that needs pulling together with cramps.

My interpretation of a sprung joint, and what I aim for, is no more than gentle squeezing of the two planks in the centre to close the gap. By gentle I mean ideally no more pressure than you might apply with fingers and thumb either side of the spine of something like a 2-1/2lb., to 3-1/2lb. book as you pick it up or carry it. Hoadley's Understanding Wood, (~2-1/2 lbs.) or Joyce's The Technique of Furniture Making (~3-1/2 lbs) fit the definition.

Using this definition two 5" wide planks each 20" long require a smaller spring than you can get away with when joining two 5" wide planks 80" long of the same cut and wood species. Slainte.

Re: Sprung joints, moisture gradient, and finish

#5

Thanks for the excellant explanation

vlparisian in Houston

>

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