As I have mentioned in several blog entries over the past few weeks - I've been experimenting quite a bit with the DevCon 2 Ton Epoxy that we carry.
I'm particularly impressed with it's clarity, superiour hold, and clear, glassy finish. Ease of use is pretty good too - as it comes in a dual action push tube, you just uncap, squeeze out two matching puddles of resin and hardener, and mix.
But I was gluing some bails onto dragons' eyes a couple of weeks ago, and for one reason or another, the twin streams of hardener and resin did not come out in equal quantities - one being quite a bit smaller than the other. I added some more - but now it worked correctly, and all I got was bigger quantities, not matching quantities. That is the down-side to this style of packaging - if it doesn't get it right, you can't really fix it.
Instead of starting again, which would have been the smart thing to do, I proceeded anyway. I mixed, applied, let it cure. After 24 hours, I noticed that it was still a little sticky - not the hard, glassy finish I had come to expect. I gave it another 24 hours, and it was still just a little yielding to pressure. Hmmm - time to consult an expert.
That expert would be my significant other - who is an expert in epoxy by virtual of building an airplane in the back yard. It is a "composite" construction aircraft - being built primarily of fibreglass and epoxy resin. Yep - the same stuff that I'm gluing bails on to glass dragon eyes. We have similar pastimes. It's just that I like to melt the glass I use, and he likes to glue it. And he mixed his epoxy up by the cup-full. (By the way - I have tried melting the glass-fibre - it goes white and devits and is intensely ugly - in case you are interested.)
Over the years, I'm sure that I have learned far more about epoxy and fiberglass than I ever thought I needed to know, and I'm sure he feels the same way about beads.
Anyway - I showed him my still slightly sticky pendants and explained what had happened, and asked what to do about it.
He told me that epoxies in general that have a 50/50 mix have a 10% margin - so you don't have to be dead-on accurate with your mix, although, obviously - in small batches, it's easier to blow the proportions. And, just as obviously - I had done so.
He suggested that I heat them to about 130° F and hold them there for about 2 hrs, and that should accelerate the cure. This is known as post curing. Otherwise, they would probably cure by themselves if just left for a couple of weeks.
He also pointed out that the epoxy would soften while doing this, so it would be best to prop the bails up, so that they didn't move and lever themselves off. If I didn't want this to happen, I should heat them up slowly. Given that I have a kiln, and can program it for very fine temperature control - I elected to do this. However, if you have the same problem with tacky epoxy, your standard household oven will give you enough heat too.
I programmed the kiln to heat up to 100° F, and hold for 1 hour, then to heat up at the rate of 120° F per hour to 130° F, and hold for 1 hour, and then shut off.
When they came out - they were still warm, and the epoxy was still slightly tacky - but once it had cooled to room temperature, it was fine.
As a control, I did not heat one of them, and just left it. It has hardened on it's own, as promised.
I asked if I could also use heat to remove the epoxy - and the answer was: "Heat it to 150° F and scrape fast." I have not, as yet, had to try this.
But, I'm sure I will at some point. ;-)
As usual - we learn more from our mistakes than we do from our successes.