Why Reversibility?

I have always tried in my career to follow accepted museum practice for proper conservation of antique furniture and other wooden objects.  I have naturally had some independent conclusions which differ from certain methods I have seen used in museum labs, such as using modern upholstery methods and materials on antique furniture, and I have discussed that issue previously on this blog.

Also, I have resisted using modern, experimental chemicals, glues and finishes, which are commonly accepted in many conservation labs.  Part of that decision is driven by the experiences I had in European workshops and museums, where traditional methods and finishes are more standard.  Part of that decision goes to the basic reason I started this career in the first place.  I was involved in some of the most high tech projects in my field, high energy particle physics, and had my "mid life crisis" at the age of 25.  I walked away from my research lab and into the 18th century.

I believe, absolutely, that the highly trained craftsmen working in the pre industrial age knew exactly what they were doing.  They had excellent materials, basic hand tools, and the experience of their ancestors, passed down over the ages.  They also had the financial support of a very rich class of merchants and royal patrons, who recognized quality at a very high level.

In any rate, the reason I am posting today is that I have recently been asked again about the reversibility of animal protein glue.  A basic rule in all museum restoration is that, whatever is done, it should be reversible.  The reason is that, in the case where future research indicates a better method is possible, the restoration could be removed and replaced with the better repair.

More importantly, it is essential in building furniture which is functional that repairs can be made when the object breaks.  Any furniture maker who thinks his creation is never going to be damaged or broken is not dealing with reality.  Objects which are designed to last centuries will be damaged.  That is a fact and that is why I never run out of work.  I can't count how many chairs I have repaired in 40+ years, and how many different types of damage they suffer.

Therefore, when a broken chair or other object arrives and there is nails, sheetrock screws, epoxy or, worse, "the strongest glue on planet Earth," I need to inform the owner that it will cost more and the repair will be less successful than if it had not been repaired or if it had just been glued with protein glue.

Fish glue, bone glue and hide glues are all reversible.  You need to understand what that means to effectively work with these glues.  They are all water soluble.  They need water combined with heat to change from solid to gel to liquid and back again.  Each has specific working characteristics, but they all have the same thing in common:  Adding heat and moisture reverses them.

Fish glue is somewhat different, as it is liquid normally at room temperature.  It cleans up easily with water.  Adding heat liquifies it easily.  It is designed to be used for holding materials together which expand and contract differently with environmental changes, like gluing metals to wood, or horn, ivory, bone and shell to wood.  It "relaxes" somewhat during heat/moisture cycles, and then sets again when stable, holding tight.

Bone and hide glues are normally dry and need cold water to be hydrated, then heated to be used.  Thus, they set initially by losing heat and then by losing moisture.  Each gel strength has a different rate of set and gel point, which is why I like to  use 192 gram strength.  It works well for all applications.

When I decided to modify my 192 glue with urea, my goal was to simply lower the gel point slightly, so that it could be used from a bottle.  The result is Old Brown Glue, which is currently carried by Rockler, Lee Valley, Tools for Working Wood, and several Woodcraft locations.

Fine Woodworking Magazine did an independent testing of OBG as well as other glues in their issue #192 (August 2007).  According to their tests, OBG was stronger in all tests than the "strongest glue on planet Earth."  I guess that makes it the "strongest glue in the universe?"  I should point out that, since it is non toxic and reversible, if you get it on your hands, just wash it off.  On the other hand, if you get polyurethane glue on your hands, you need to wait for several months while the skin falls off and you grow new skin.

That fact alone makes reversible glue attractive.

Here is the link for the pdf of the Fine Woodworking Test:

"How Strong Is Your Glue?"

If you are in Pasadena this weekend, stop by our booth at the Convention Center.  We are participating in the Woodworking in America show, and you can pick up some of our glue and see a French chevalet in action.