Can Biscuits Telegraph Through the Wood Surface?
George VondriskaDescription
Woodworkers have said that biscuits shouldn’t be used in thin wood (½” or less thick) because when the biscuit swells with moisture from the glue, it’ll exert enough force that the shape of the biscuit will telegraph through the surface of the wood. Fact or fiction? Let’s find out.
How biscuits work
You’ve probably noticed that when you insert a dry biscuit into a biscuit slot, it’s kind of loose in the slot. You don’t get a good fit between the biscuit and the slot until you add glue. Moisture from the glue causes the biscuit to swell and fill the slot.
How the experiment was done
This was a simple test. The question was whether biscuits swell and telegraph through the surface when used in thin wood, so we grabbed some thin stock. Three species were used; pine, cedar, and red oak. Pine and cedar were very intentional choices because they’re relatively soft woods. You would think that if a biscuit can make wood move, it’s more likely to happen in soft woods than hard woods.
How thin?
The six boards used were planed to just under ½”. This thickness is generally called out as the minimum for biscuits—the point at which telegraphing starts.
And so…
In the end, no change was seen in the surface of any of these boards. Even the soft woods—the pine and cedar—showed no indication that biscuits were in the seams.
Biscuit joiners are great tools
Biscuit joints and biscuit joiners are great, and can add a lot to your woodworking. They really simplify joinery. There are lots of great ways to use these joints.
I feel like I'm on "MythBusters" here, looking at this idea of if I put biscuits in really thin wood do they telegraph? So here's the thing behind this. A lot of times when you read about using biscuits for wood joinery, you'll hear, you'll read, you'll see that you shouldn't use them in really thin stock. And the premise is that when we put a biscuit in the biscuit slot, and then we add glue, they do absorb glue. They absorb moisture out of the glue. So what happens is that causes the biscuit to swell. So what you may have experienced is that when you cut a biscuit slot, they're a little bit loosey-goosey in that slot initially, but they're gonna fill the slot once you hit them with glue, and they get the moisture from the glue. So to take that a step further, if we've got that number 20 biscuit in a piece of wood that's really thin, I have heard people say, "You can't do that because as it swells, the shape of that biscuit is gonna telegraph through the surface." So to test this out, I've got a bunch of stuff here, red oak, pine, and cedar that I planed to just under a half inch. It's not as thin as 7/16, but it's less than a half. And then where the lines are, I cut slots for number 20 biscuits, same way I always would, I glued those in, and then I gave that a couple of days. And what we're looking for is do I have a football shape in the surface? And even with a straight edge on here, the spoiler alert is the answer is no. I guess it's not a spoiler alert, 'cause that's what we're here to talk about. Even laying a straight edge on here, that surface is still dead flat. In the red oak, it doesn't surprise me 'cause that's a fairly hard wood, but honestly, I'm a little surprised that in the cedar and the pine, which are both pretty soft, at this thin thickness, at such a minimal thickness, that with the potential pressure that's coming from that biscuit expanding, that there's nothing here on either face. And you can tell from the glue drops, I did nothing after the glue up here. This is still the way it came out of the clamps. So your mileage may vary, always gotta qualify these things, but in my reasonably scientific little experiment here, putting number 20 biscuits with conventional wood glue in really thin, soft wood, I didn't get anything telegraphing through the surface. So I think we're safe to do this.
Who uses biscuits anymore? 🤣
How about trying another round with 1/4" or 3/8" stock? Just for giggles...
I have had plywood plies separate at an edge to edge joint due to biscuit swell. The material was 3/4" exterior grade douglas fir plywood.
I'm not saying you wrong "but" I'm sure I saw an article and I'm fairly sure it was in FWW magazine in the last ten years ago showing a finished dark wood table with obvious dips centered where the builder said he put biscuits. He contended that the wood had swollen from the glue then continued to shrink after he had done his final planning, sanding and finishing. I tried to find the article but their search is awful. It just gives you its best 600 guesses of what it thinks your looking for. Thanks George
I think any glue joint has the potential to telegraph if you sand it down too soon. The wood swells from the moisture in the glue and if you sand that down before the moisture equalizes you end up with a valley along the glue line as the panel comes to equilibrium. Even in a simple long grain butt joint with no splines or biscuits
Many years ago this biscuit topic came up with Norm Abrams on the New Yankee Workshop. He stopped using them because over time there was a discoloration at the biscuit location.