Glue wants "dry" parts but biscuits must be moist??

[Stephen Tashiro]

Many glues say that they should only be used on parts that are "dry". Wood biscuits are supposed to be moistened before inserting them. How are these two ideas compromised? Is there enough moisture in some glues to dampen the biscuit without having to apply additional moisture?

( It's interesting that urethane glues like Gorilla Glue say to moisten porous parts before they are glued.)

[Harvey Melvin Richards]

Many glues say that they should only be used on parts that are "dry". Wood biscuits are supposed to be moistened before inserting them. How are these two ideas compromised? Is there enough moisture in some glues to dampen the biscuit without having to apply additional moisture?

( It's interesting that urethane glues like Gorilla Glue say to moisten porous parts before they are glued.)
I will moisten wood and biscuits if I'm using Gorilla glue. I usually use a spray bottle. For all the rest of my work I use dry biscuits, and put glue in the slots using a Lamello glue bottle.

I have used Gorilla glue since it first came on the market, and I'll have to say I'm not that impressed. Glue joints that I've had to redo failed miserably. I almost always use Titebond if I can.

[Ken Fitzgerald]

I use Tite-Bond and I don't moisten the biscuits. I apply the glue but even then, if I'm using multiple biscuits and am a little slow getting them in the slot and the two pieces clamped together, those biscuits will swell. I can only imagine what would happen if I moistened the biscuit before putting a urethane glue on them.....

[Steve Griffin]

I've used cases of Biscuits over the years, and not a single time "moistened" them. Sounds like trouble to me.

In fact, one of the biggest mistakes you can make is over gluing them. If you put a big puddle of glue in the slot, you run the risk of getting biscuit shadow--the wood will swell with the moisture in the glue and you can see a faint outline of the biscuit on the surface. Not the best look for a heirloom dining table...

My latest theory is to brush glue on the edges of a glue up, and nothing in the slots other than the drop or two that oozes in.

In only takes about a drop of glue to hold a biscuit in so well it is impossible for even a super hero to remove it.

-Steve

[John Coloccia]

I've never heard of moistening biscuits with anything but glue. That's moist enough.

[Paul Murphy]

Many glues say that they should only be used on parts that are "dry". Wood biscuits are supposed to be moistened before inserting them. How are these two ideas compromised? Is there enough moisture in some glues to dampen the biscuit without having to apply additional moisture?

( It's interesting that urethane glues like Gorilla Glue say to moisten porous parts before they are glued.)

The polyurethane glue cures in the presance of moisture, so if you live in a very dry climate and your wood has dried to less than 8% of moisture then you might mist one surface, and spread glue on the other surface.

http://www.woodworking.com/ww/Article/Some_Observations_On_Polyurethane_Adhesives_7524.a spx

[Will Overton]

Many glues say that they should only be used on parts that are "dry".

I don't think that's true of most wood glue. Gorilla glue and it's ilk require some moisture as a catalyst, but I would never be using it in conjunction with biscuits.

[Philip Rodriquez]

Instructions lessen the imagination :cool:

[Stephen Tashiro]

Instructions lessen the imagination :cool:

That's true except when you start imagining "I wonder what could happen if I don't do that".

[Curt Harms]

I don't think that's true of most wood glue. Gorilla glue and it's ilk require some moisture as a catalyst, but I would never be using it in conjunction with biscuits.
As far as I know, the object is to put some glue in both halves of the biscuit slot. Then insert a dry biscuit and clamp up. The glue moisture will cause the biscuit to swell and make a tight joint. I tried polyurethane glue one time on biscuit joints. It came apart real easy a couple years later. Biscuit joints glued up with titebond and the like hold very well. Cut a biscuit joint in two pieces of scrap, glue it up with titebond and gorilla glue, let it sit a few days and cut it apart. See which one looks better.

[Paul Murphy]

As far as I know, the object is to put some glue in both halves of the biscuit slot. Then insert a dry biscuit and clamp up. The glue moisture will cause the biscuit to swell and make a tight joint. I tried polyurethane glue one time on biscuit joints. It came apart real easy a couple years later. Biscuit joints glued up with titebond and the like hold very well. Cut a biscuit joint in two pieces of scrap, glue it up with titebond and gorilla glue, let it sit a few days and cut it apart. See which one looks better. This is a situation where you probably should have added some moisture. Polyurethane is not gap filling, and cures in the presance of moisture. If there is no moisture to swell the biscuit, there is a gap. If there is less than ~8% moisture, there is not enough moisture to properly cure the glue.

From the article at this link:
http://www.woodworking.com/ww/Article/Some_Observations_On_Polyurethane_Adhesives_7524.a spx
"Many of my tests in the shop involve trying to determine what happens to a bisquited joint when glued with poly. By way of explanation, after cutting the slots and applying the adhesive in the slot and on the surface grain of the components, I dunk the plates in a container of water and remove them right away. I then place the plates in their slots, assemble the components and clamp for the appropriate period of time. I try to plan my assembly and glue-ups so I can leave everything clamped overnight. This generally works for me, but may be a little overkill for those in higher humidity locations. After the adhesive has cured, the clamps are removed and I subject the joint to varying degrees of stress. Sometimes the assembly is placed in a vice and struck with a hammer. I have found the joints to be on a subjective par with joints assembled with PVA adhesives. More importantly, some of the samples are sawed across the joint location , so that the space between the bisquit side and the wall of the slots can be observed. My fifty-six year old bifocal aided eyes have thus far, be unable to discern a glue line, much less unfilled space. A 6X power, cheap magnifying glass, also fails to reveal any space at the location. The excess space at the ends of the bisquits are usually filled with cured adhesive. When I try to remove the bisquit portions by levering with a scratch awl, I find the plate material comes out in very small portions, sometimes bringing some of the wood from the wall of the slot with it. This compares to the same general results with joints assembled with PVA adhesives."

I have used polyurethane glue with this technique, and it works well for me. I don't use it often, but it has stood the test of time for me. One nice thing about polyurethane glue is it has a very thin glue line with no "creep".

[Curt Harms]

Paul you may be right about this but too much moisture with polyurethane glue results in mucho foam and little strength. I suspect a little water misted on the biscuit would be okay but it's just as easy to use a PVA glue for me. I do recall manufacturers of polyurethane glue cautioning against using too much water and getting a weak joint.

[Chris Padilla]

In fact, one of the biggest mistakes you can make is over gluing them. If you put a big puddle of glue in the slot, you run the risk of getting biscuit shadow--the wood will swell with the moisture in the glue and you can see a faint outline of the biscuit on the surface. Not the best look for a heirloom dining table...

Yes, this is well to note for folks using lots of biscuits. First judge if you really need to glue the biscuit in the first place. I would avoid purposely putting glue on them if it wasn't necessary. If some gets on them, no biggie but I wouldn't saturate them unless necessary (like perhaps in a non-hardwood to hardwood/non-hardwood situation: say oak edging on MDF).

That localized swelling Steve mentions will go away but only after you let several days go by for the wood to rebalance its moisture content. If you sand too soon, you could end up with dimples there later.

[Paul Murphy]

Paul you may be right about this but too much moisture with polyurethane glue results in mucho foam and little strength. I suspect a little water misted on the biscuit would be okay but it's just as easy to use a PVA glue for me. I do recall manufacturers of polyurethane glue cautioning against using too much water and getting a weak joint.

Correct about a little water. I took a damp sponge and quickly wiped the board without glue, dipped the biscuit into water and placed it in the slot, and clamped the 2 boards together.

Just trying to help folks avoid problems, thanks for understanding.

I also want to point out what I think are very nice features of polyurethane glue:
1) A very thin glue line. If you match the grain of two boards well, you will have a hard time finding your own glue line.
2) No "creep". If you were to edge glue a quartersawn board to a flatsawn board with pva glue you will get a seasonal difference in thickness that results in a non-flush glue line sometime during the year because pva creep allows the fractional movement. Polyurethane will not allow this fractional "creep".
3) Long open time. If you have a complex and unforgiving glue-up, polyurethane will give you more time to get it completed.
4) A very high solvent resistance. For example, I had a couple sample boards from project cutoffs that I was staining with alcohol soluable stain to test for coloring that I liked. I had used all of the 2 cutoffs, and was washing the stain off with alchohol to try different colors. The pva glueline was softened by the alcohol, while the board with a polyurethane glueline was unaffected.

I use yellow glue most of the time, but there are times when polyurethane glue makes something easier for me to do. Keep it in mind for those situations, you might find a use for it.

[ken gibbs]

Steve,

Do not premoisten biscuits. Use Type three white glue. You can wipe off the glue line if you see glue beads squeezed out of the joints when you apply your pipe clamps. Buscuits are made to expand after being inserted and get pleanty of moisture when you wipe off the seams with a damp spunge.

[Curt Harms]

Correct about a little water. I took a damp sponge and quickly wiped the board without glue, dipped the biscuit into water and placed it in the slot, and clamped the 2 boards together.

Just trying to help folks avoid problems, thanks for understanding.

I also want to point out what I think are very nice features of polyurethane glue:
1) A very thin glue line. If you match the grain of two boards well, you will have a hard time finding your own glue line.
2) No "creep". If you were to edge glue a quartersawn board to a flatsawn board with pva glue you will get a seasonal difference in thickness that results in a non-flush glue line sometime during the year because pva creep allows the fractional movement. Polyurethane will not allow this fractional "creep".
3) Long open time. If you have a complex and unforgiving glue-up, polyurethane will give you more time to get it completed.
4) A very high solvent resistance. For example, I had a couple sample boards from project cutoffs that I was staining with alcohol soluable stain to test for coloring that I liked. I had used all of the 2 cutoffs, and was washing the stain off with alchohol to try different colors. The pva glueline was softened by the alcohol, while the board with a polyurethane glueline was unaffected.

I use yellow glue most of the time, but there are times when polyurethane glue makes something easier for me to do. Keep it in mind for those situations, you might find a use for it.

Another place polyurethane works at least somewhat is gluing new work to existing already finished work. I found myself building a base to an already in service entertainment center. How to fasten the transition molding without a bunch of fasteners and attendant holes? I wiped with mineral spirits several times over a few minutes to remove any wax then glued the transition molding to the lower portion of the cabinet. I wanted the top to be removable for moving the piece so I just glued to the already finished part. It's been a few years and nothing has moved :).