[Kristine Madras]

Good Afternoon!

We recently had one of our wooden, antique butter molds break drop and break. I was wondering if acid-free carpenter’s glue is ok to use to repair it, or if I should stick to fish glue?

Thank you!

Kristine M.

[Rebecca Gridley]

Hi Kristine!

I am the monitor for this week and would be happy to help you with this inquiry.

Carpenter’s glue is a PVAC adhesive (polyvinyl acetate), much like the familiar white Elmer’s glue. Repairs made with this type of glue can be very difficult to reverse — and I can speak to this from personal experience! These glues do not have great aging properties, and actually become stronger over time. This is not ideal, as the glue may become stronger than the wood itself. As wood is a hygroscopic material, it will react to the surrounding environment by expanding and contracting slightly. If this happens, this super-strong join will not budge. This could introduce new stresses to the surrounding area and result in the formation of cracks. Plus, we don’t know what additives are used to make carpenter’s glue “acid free,” so it is difficult to know what impact these additives will have on how the glue ages.

Fish glue is also a hygroscopic material, and is reversible in water. Given the option between the two, I would recommend using fish glue — any future conservator who needs to reverse the repair will certainly thank you! Without knowing any details about the piece other than your brief description, I think it should be strong enough to get the job done.

I hope this is helpful. Please let me know if you have any additional questions or would like to share a photograph to review!

Rebecca

[Evelyn]

I agree, I have never found a good substitute for fish glue. Unfortunately it is getting harder to find.

[Marc Williams]

Good old Lee Valley Tools has had it for decades and still does. http://www.leevalley.com/US/wood/page.aspx?p=20019&cat=1,110,42965,20019

To add to the discussion of why fish (or all hide [animal protein] glues) works best for wood other than its’ proven reversibility with water even after hundreds of years, I will attempt to dredge up from my mind info from the course “Adhesives for Wood Conservation” taught partially by adhesive chemists I took quite some time back. Hide glues form an actual molecular bond with the wood itself. In order for them to form the strongest bond, the glue layer needs to be as thin (but fully coating all surfaces) as possible. Properly done, the bond is as strong as the wood itself. For wood in specific, it is as strong a glue as there is. PVA glues in comparison form only a physical bond with the wood, that is they interlock with the interstices on the surface of the wood but do not form a molecular bond. As such, in most instances, they are less strong than hide glues for wood. These comments are specific to wood. Other substrates will behave differently.

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