The glue I’m using says it dries in an hour but cures in 24 hours. What is the difference and is there benefit in leaving clamps on for the full 24 hours? Assume strength is important.
asked Jan 22, 2020 at 2:56
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1Very similar to with paints and varnishes, which is covered in woodworking.stackexchange.com/questions/884/…– GraphusCommented Jan 22, 2020 at 9:18
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1It is, but I thought the differences were important enough to highlight. I'm finishing up a project now and the varnish question didn't really tell me "removing clamps after X hours is fine", which someone with experience might answer with. Based on info elsewhere on the web, I've now left it clamped for 24h just in case.– Richard WatsonCommented Jan 22, 2020 at 14:15
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BTW I had read your comment here woodworking.stackexchange.com/questions/6465/… - you mentioned 6-8 hours/overnight and early removal possibly opening up a gap and weakening. I still don't know if the 24h curing time is relevant, but I assume so and went with your advice to be cautious.– Richard WatsonCommented Jan 22, 2020 at 14:39
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2What glue? Brand name and details, please, or this Q&A is guesswork. There are lots of different types of glue.– user5572Commented Jan 22, 2020 at 16:29
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Does this answer your question? What is the difference between "curing" and "drying"?– KromsterCommented Jan 24, 2020 at 5:05
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2 Answers
What is the difference [between glue drying and curing time]
As covered somewhat in this previous Q&A, How long does glue take to dry? all PVA wood glues 'dry' by literally drying initially and then there's a curing phase, where they undergo a chemical/structural change and harden more completely and get closer to their full strength.
So your glue says it dries in an hour and takes 24 hours to cure, so it's just holding the wood together at hour one and closer to being at full strength after 12, and has nominally achieved max strength after a day. But remember those times are averages, based on assumptions of temperature and humidity and wood moisture content. If any or all of those are different where you are your glue dry times will be different. Even using air-dried wood rather than kiln-dried wood your drying times will differ slightly, in the same workshop on the same day.
It's because you can't always know for sure what you're dealing with that you err on the side of caution, unless you have no choice. So it is good general practice to leave clamps on longer than you think is strictly necessary if you can. If there's nothing other than your impatience urging you to remove your clamps and go on to the next step resist the urge!
is there benefit in leaving clamps on for the full 24 hours?
Rarely. However, there's no downside to it either (other than the clamps being tied up and not available for other uses).
To put this more generally: it doesn't hurt to leave clamps on too long, but can if you take them off too soon.
As I cover in my previous Answer, if you take clamps off too early the glue joints can open up very slightly and result in greatly reduced joint strength. But one detail I wanted to add is that even an invisibly small shift in position or opening between boards can have a profound impact. So your joints don't have to be noticeable open or out of alignment for them to be weaker than they could be — which is literally stronger than the wood around them. This is the goal to aim for, whether the workpiece in question needs it or not. Because, why not?
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Thank you. I was doing a lot of reading about diminishing returns but you capture the gist of it. I would imagine a curve of hours to strength world be an obvious guideline but it seems a rare idea. I saw some info dating back to almost WWII. Commented Jan 24, 2020 at 19:30
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1Yes a lot of the research was first done in the early part of the 20th c. (this is when high or very high clamp pressures being desirable was first discovered, amazing that it's taken 80-100 years for this to filter down to leisure woodworkers)!! There will have been much new testing since then, including confirmational rechecking of older tests, but a lot of the data will be proprietary and we'll never get access to it, although we get the benefit of some of the conclusions in dribs and drabs.– GraphusCommented Jan 25, 2020 at 8:12
From the Titebond website:
What is the clamping and drying time of Titebond Wood Glues?
For most of our wood glues, we recommend clamping an unstressed joint for thirty minutes to an hour. Stressed joints need to be clamped for 24 hours. We recommend not stressing the new joint for at least 24 hours. For Titebond Polyurethane Glue, we recommend clamping for at least forty-five minutes. The glue is completely cured within 6 hours.
Keep in mind that higher temperature, drier wood, and lower humidity speed drying time; while lower temperature, wetter wood, and higher humidity slow it. Stressed joints would be things like a bent lamination.
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1It's also important to note that if you're using a vacuum press for things like bent lamination that some glues actually have a maximum clamping time as they require oxygen to cure. Commented Jan 22, 2020 at 16:48