Hammer Veneering with Hide Glue: Lessons Learned
|Figure 1. A homemade veneer hammer|
This is a quick compilation of everything I've learned about doing hammer veneering, both from the radio cabinet project, research, and my previous experiences with veneering and hide glue. Pictures are ones I've compiled from different projects (mostly the radio cabinet piece) to show what I'm yammering on about, so it's not quite like following a project. I've not done copious amounts of veneering, but I've done some over the years and have researched it quite a bit - so here's everything I know in one place (this should be short!).
Hammer veneering with hide glue, though considered daunting by many, is actually pretty easy. It's really just a matter of preparation... The procedure I found that seems to works best for me is a traditional method...
|Figure 2. Plans for a veneer hammer|
First off, hammer veneering is inexpensive, especially when you compare it to techniques like using a vacuum bag. There are no expensive tools to purchase - a veneer hammer (pictured in Figures 1 and 2) is easy enough to make, taking all of about an hour at most. A hammer veneer, optimally, is a strip of brass held in place in a wooden handle. You can even use a solid wood hammer (I would use a closed-cell wood like maple), but a metal one will glide on the cooling glue longer, essentially giving you more working time, where a wood one will begin to stick sooner. Also, the metal can be heated with a heat gun to further extend your working time, and hold that heat longer...
Next, hot hide glue is the way to go. I always see suggestions for things like contact cement and the like using cauls instead of a veneer hammer, and it always makes me cringe. Hide glue is simply better,and with Stephen Shepherd's book on the subject, ignorance of it is no longer an excuse. After that, it's just a matter of following some simple steps:
|Figure 3. Sizing helps to keep the veneer from curling too severely when gluing. Here, only one side has been sized ad you can see the results|
1. Preparing the Veneer: It's not necessary for most standard, stable veneers but if it's an unruly one that won't lay flat and is prone to cracking (e.g. a burl, highly figured, or crotch veneer), and you have some time (like a week before the actual event), sizing (figure 3) your veneer with a very thin mixture of hide glue can help flatten it and make it so the veneer doesn't suck up all the moisture and curl (at least not as much) when you start veneering. It's also a good way to use up older hide glue whose strength is suspect...
Another option is a veneer softener - softeners can be used the day before often are some form of glycerin or glycol.
Store the veneer between some plywood sheets and wax paper to help flatten it, and only take it out when you are ready to use it.
2. Veneer Both Sides: Unless the panel is *firmly* mounted, you *will* need to veneer both sides - even if it's been veneered on both sides previously. Even if it is firmly mounted, its still advisable to remove the top and veneer both sides if possible. It's absolutely amazing the strength the warping wood will exhibit if veneered only on one side... The imbalance of moisture applied heavily to one side during the process will set the warping process in motion and will ultimately ruin all the work you've done. Fortunately, you don't have to veneer both sides the same day - you can veneer the back side late one afternoon and the top side early the next morning. But the sooner the better - don't let it sit too long, as forcing a warped board to flatten out is likely to crack your freshly applied veneer.
|Figure 4. A simple hot hide glue setup is inexpensively assembled|
3. Make sure you prepare enough glue; I have found that a small mason jar - about the size of a 16 oz. jelly jar - will give you plenty of glue to do a top about 18" x 24" with enough left over if you need to reapply some. Remember you can refrigerate or freeze the leftover glue so it doesn't go to waste.
4. Warm the pieces you are gluing if the weather is cool with a heat gun. Hide glue is very sensitive to temperature when you are using it - try to have the shop as warm as you can also - working in a cold environment on cold material will severely limit the open time of the glue.
5. Apply the Glue: Working quickly, brush a good coat of glue on the panel that's receiving the veneer, then on the back of the veneer - then on the top of the veneer. Yes, the top - the glue on the top acts as a lubricant for the veneer hammer (which would be better called a veneer 'squeegee').
One of those dedicated hide glue brush would be nice to have, but I've made due with a synthetic brush from a set purchased at the dollar store. You don't want the glossy, smooth bristle brushes, get the ones that at least look something like animal hair...
|Figure 5. Hide glue from toolsforworkingwood.com|
Hide glue is available from several sources - one of the best sources I've found is Tools for Working Wood, where they have hide glue available in a variety of strengths, measured by gram strength. All you really need to know about gram strength is that the higher the number, the stronger the glue - and the shorter working time it will have.
For veneering, a 192 gram glue strength is a good choice. The 250 gram is stronger, therefore better for structural gluing - but the 192 will give you more open time and veneer doesn't require the higher strength. When mixed, it should have the consistency of store-bought syrup or honey at room temperature - when you pull your mixing stick out of it the glue should run off the end of it for a few seconds before beginning to drip. Most experts agree that using glue prepared the previous day (and allowed to cool overnight and reheated) is optimum - however I've found I can use freshly prepared glue and older glue successfully. When I say older, I mean up to 3 unrefrigerated days old. On that subject - I wouldn't use glue that's been refrigerated (not frozen) for more than 7-10 days, but I have successfully used glue that had been frozen for 3 years (I found a forgotten bottle of hide glue buried in the freezer when cleaning it out). However, I would not used glue that's been through more than 3 full freeze-thaw cycles.
6. Hammer Veneering: Now that you have applied the glue to all surfaces, put the veneer in place onto your panel. Using your hammer in a zig-zag pattern, starting from the center out squeegee the veneer flat onto the top, working out the air bubbles and excess glue between the veneer and the panel, and repeat. As the glue cools, it will begin setting up and the veneer will begin to fully adhere to the base panel below. You can tell if the veneer is adhering by the sound of the veneer hammer - before the veneer fully adheres, the hammer will have a "crackling" sound to it. As the glue cools and gels, and the veneer begins to adhere, the crackling will be replaced by a more "solid" sound as the veneer and it's substrate become one. Once you are satisfied the glue has set up, you can stop hammering.
|Figure 6. Illustrating the jointing process. Top: the overlaid joint. Middle: cutting through both layers of veneer. Bottom: removing the waste.|
I should mention here the options you have if you are veneering an area larger than your veneer. There are two methods used for jointing veneers together - laying one down at a time with a slight overlap as shown in Figure 6; or using veneer tape to hold the two veneers together dry and then laying them both down at once. I've only tried the former once, with poor results - mostly because I used a wooden ruler - you can read about my adventures with that here.
Either method will work, I think. I prefer the tape method, at least when I can get away with it, but I'm quite sure there are situations where overlaying them would work better.
7. Extending working time: An additional trick to extend working time is keep a spray bottle of heated water available (one floating in you glue pot is handy). You can lightly spray the heated water to help lubricate the hammer. Don't do too much of this, as too much water will weaken the strength of the glue.
As a last resort you can re-apply some hide glue to the surface. Try to avoid this if possible, but if you have some parts that are adhering unevenly and leaving a bubble, re-applying some glue can temporarily reactivate the glue underneath and provide additional lubrication for the hammer. Be warned, however, that there is a stage in the gluing where doing this probably does more harm than good...
8. Final Act: Once the glue has set up, but before its as hard as a rock, it's a good idea to lightly scrape the excess glue off of the surface while you can... Then, I like to cover the veneer with wax-paper and place weight on it. Not so much for holding the veneer down as to slow the drying process, as if it dries out quickly it can dry unevenly - the top of the veneer dries out more than the substrate, which can introduce stresses on the veneer that can lead to cracking. By placing wax paper over the veneer, the drying process is more even throughout.
|Figure 7. A Stanley #80 is a great tool for removing the majority of the glue from the surface.|
9. Prepping the Freshly Veneered Surface: Once the glue has dried sufficiently - at least 24 hours, you can begin preparing the top for finish.
Start by trimming the veneer to size - there's inevitable some that hangs over what you want as a finished edge. Depending on the veneer, it may be better to start cross-grain first, and finish with the grain to avoid tearing a little sliver out right at the corner of the work. Speaking of which, at the edges don't work outward - always work toward the interior of the piece to avoid taking a section of veneer with you. A sharp knife or veneer saw work well, or a flush trim or profiled bit in a router if you so desire.
Scraping is the preferred method for removing the excess glue from the surface, and using a cabinet scraper (Stanley #80 style) or scraper plane (Stanley 112 style) is the way to go - you can do it with a hand scraper but that method requires a great deal of effort. I usually reserve that method until after I've scraped the layer of hide glue off of the surface of the veneer and it's down to bare wood again. As you are scraping, inspect the veneer for bubbles and cracks in the veneer and repair them when you you can. Smaller bubbles and cracks can wait for later, but large ones can tear out if you are not careful.
One always wants to try scrape into the edge and not over it to avoid tearing out chunks of veneer right at the edges. As you get down to bare wood, reduce the cut the scraper is making, and switch to hand-scraping as soon as you can so you don't chip or go through the thin veneer, especially at the edges where it's most vulnerable.
You'll see a color change in the veneer as you hit pay-dirt (wood), and be careful when you do - veneer is very thin and it's easy to work right through it. When you do (and its likely you will, I think everyone does it at least once in their career) the only real solution is to start over.
I hope this summation has been of some service...