Biscuit Depression? – Question of the Week
This week’s question comes from Jim. He writes:
“I just finished watching all your videos and I have a question about the table top you did in Episode 11. A little while back I saw a episode of New Yankee Workshop where Norm said he stopped using biscuits in table tops because they found that later, depressions showed up in the tops from the biscuit slots. Have you seen this or heard of it happening before?? Oh, and love the videos and all of the site. And now I’m on Lumberjocks as well…thanks to you. Keep on keeping on :>)”
And here was my reply:
“Hey Jim. I have heard Norm say the same thing but I have never actually seen it in my work. As I understand it, the theory says that once the joint is closed up, you essentially create a nearly air-tight pocket. This means the glue takes a long time to cure and the wood fibers around the biscuit swell. Then of course, you do your final sanding for the top and flatten everything out. As the glue cures completely over the next few days (weeks?), the wood fibers shrink down a bit and create a sunken area visible from the top surface of the table. I will occasionally pass that warning on to my students as a “heads up”, just in case. But in reality, I have never seen it happen. So I still use them for alignment now and again. Hope that helps.”
So this week’s question is more like a survey than anything. I want to hear from you, the readers, about your experiences with biscuit depressions. Are you a victim? Tell us your story!
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Does this depression happen on all thicknesses of wood or only thinner pieces? I would think that if it is centered in a 3/4″ or greater board with >1/4″ of wood on ether side that the fibers would be strong enough to prevent that type of movement. Maybe the tightness of the grain and flexibility of the wood fibers of different species come into play…
I haven’t had it happen to my projects . Norm more recently has said you don’t need anything if your boards line up well.
I do have a Cherry End Table with biscuits that is 2 years old and shows no signs of indentation.
Good question I always wondered what others experiences were for this issue. I look forward to reading them in this great blog.
Kip
From my understanding its the moisture content of the wood, not the biscuit swelling.
When you use biscuits you use more glue and introduce more moisture to the joint, causing the wood to swell where the extra moisture is.
If you take a moisture meter to a fresh glue-up you will be able to find the areas where the biscuits are fairly accurately.
If you let the moisture in the glue-up equalize/dry out a little before sanding you will alleviate the problem.
How long depends on how dry the wood was to begin with, the type of wood, the humidity in your shop, etc…
Personally I have not had any problem with using biscuits since I have a tendency to go for several days between gluing up panels and doing any surface work on them.
I’d heard this could happen if your biscuit was close to the surface (i.e., not as much of a problem on thicker stock). No personal experience with this.
I have the feeling the “Domino Secret Police” caught up to ol Norm, ya think?
-Ace-
I haven’t seen it in about 4 tables (2 mahogany, 2 walnut) and 2 cabinets (poplar).
Maybe it happens when your biscuits have the blues. ;-)
I have never had a problem with depressions over a biscuit joint. But then I have a tendency to procrastinate, especially when it comes to sanding. I have been known to go days even weeks before sanding down a project.
I have actually seen this on some of my projects. They were recipe card holders which I glued up 3/4″ material using biscuits then planed the glued up panels to 1/2″ thick. I was looking at one of the projects a few years later and noticed a slight football shaped depression.
My assumption was the biscuit area had swollen during glue up and I planed it flush before the area dried out completely. Over time the biscuit dried creating the depression Norm was talking about. I’ve used bicuits in 3/4″ material which stayed 3/4″ and only received a sanding after glue up. None of these have exhibited depressions. It’s my opinion the only time you need to worry about this is when removing alot of material, such as planing after glue up.
With that said I don’t use biscuits for edge glue ups any more as they end up being more of an alignment hassle than they’re worth.
Wes
Does a tabletop (especially a amaller one) need the glued biscuit for strength? Probably, just for alignment. I would not plane or sand the tabletop until you are confident the biscuits are completely dry. If you work in an unheated garage like I do, that would take some time.
I actually wanted to tell everyone about a thing that I did that will maybe help someone in the future. I built a small dinning pub table for my kitchen about 3ft X 2.5ft and the boards were each around 6 inches quarter sawn and glued together. The wood that I used was oak that I air dried for about 5 months (I got it Cheap!). I knew that the top would continue to shrink (because that’s what all the experts say, lol) I was kinda curious what would really happen though ( my scientific lab experience, I couldn’t help myself). It was interesting what happened.
Since it was Qtr sawn the table didn’t buckle or anything but the interesting thing was when I glued it, the glue bonded the fibers of the boards at a specific point. Then the boards shrank slightly but where the glue was it remained the same height so if you run your hand over the top your hand feels like it’s doing the wave. I will probably just take it off one day and hand plane it and stick it back on. Looking back on it I know why it happened but when I was building it, it never crossed my mind that the seems wouldn’t shrink.
Lata
Rob
I haven’t had it happen in the 15 or so larger tops I glued up (I don’t use them on smaller tops). But if someone wanted to take the time and had some “extra” pieces of different wood species, they could glue up different thicknesses using biscuits, leave them sit around for awhile, then cut them through the joint and see what happened.
Dean
What do you guys consider a “small top?” I have glued up tops with two boards (12″ total width) and three (18″ total), and used biscuits on both. I have used them on face frames also with small frame boards. Is this a waste of time??
INCASE OF BISCUIT STORAGE WHICH CONDITION REQUIRE????????
Just saw this older post: – YES – biscuit depression is real! (or is that swelling:)
A few years ago, I made a small table top out of 1″ black walnut (top was about 14″ wide by 42″ long). It was my first project using biscuits in the glue-up. I didn’t let it sit around too long before I finished it – a rubbed penetrating oil finish with very good sheen. I let the table top sit in my shop while I finished a redwood base for it in a wine cellar, so some time passed, probably a few weeks.
Where the walnut top was sitting in the shop allowed me to see light reflected from the surface as I went in and out of the shop. I was in denial for a while – but finally had to admit that there were depressions in my “flawless” hand rubbed surface. I kept staring at the top and it dawned on me that the depressions were where the biscuits were. I made a mental note to let any future glue-ups with biscuits really dry and get balanced in moisture content before final finishing.
1″ thick walnut is pretty decent thickness for a panel and walnut is definitley hard – it still didn’t stop the top from swelling in the vicinity of the buiscuits – enough that it showed after the swelling went away.
My last walnut table had a similar thickness top and a shelf all joined with the same biscuits. This time I used my moisture meter to confirm that the wood was stable before I finished the wood. No problems with the last project!
Thought I would share this experience even though this is an older post.
Louis