Removing emulsion from old glass plates for re-use

I have a number of old glass plates (half-plate size) with old emulsion (used and developed) that have been failures at the time.
I'd like to re-use them for re-coating but what's the best way of stripping off the old emulsion?
Hot water alone isn't always successful - presumably because of hardeners in the emulsion.
I've tried searching the threads with umpteen variations on the search but with (very) limited success.
I don't fancy boiling caustic nor sulphuric acid / bichromate.
As the plates are already the right size and thin enough to fit all my plate holders it seems a shame to scrap them.
Any suggestions?

Oh, that's easy. Soak them for a while in an alkaline solution. This can be a couple of tablespoons of sodium carbonate in water, or some pellets of lye (sodium hydroxide). I generally use the latter. Let set for an hour or so and then scrape off the softened emulsion with a plastic or wooden tool. The process is easier if you warm the solution; it doesn't have to boil or anything. Nice and hot suffices.
Another way is to use household bleach, but I prefer lye because of the stains bleach tends to give.

I always use household bleach, sodium hypochlorite. It will dissolve gelatin on glass. It is fast and less dangerous in my opinion.

Btw, I have something to add to Koraks's suggestion, lye and sodium carbonate will work as he suggests. But, glass can be etched by sodium hydroxide. Although, the reaction is super slow at room temperature between glass and sodium hydroxide. It is a possibility if you have to leave the plates in that solution for prolonged periods, something to keep in mind.

Yes, indeed, lye can etch glass, but it takes much longer than this would be a concern.
I was cleaning a plain glass bottle that had held 10% sodium hydroxide for a few years. The bottle cleaned out just fine with no damage to the glass. I suspect the glass etching issue is often overstated.

koraks said:

Yes, indeed, lye can etch glass, but it takes much longer than this would be a concern.
I was cleaning a plain glass bottle that had held 10% sodium hydroxide for a few years. The bottle cleaned out just fine with no damage to the glass. I suspect the glass etching issue is often overstated.

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My experience was different. I kept the parts of my thiourea based sepia toner in separate amber bottles. My %10 sodium hydroxide left a clear mark inside the amber glass bottle. May be due to the color of the glass the micro etching more visible in my case.

It's certainly possible! I would limit the soak period of the glass plates to a couple of hours, or overnight or so. That should more than suffice without doing any damage.

Thanks for the replies chaps.
I'll try bleach first and then carbonate - leaving the lye till last.
Thanks again

Perfect; see how it goes.
I never liked the bleach much; I did use it once or twice for stripping emulsion off of xray film. Worked well, but as I said, the stains....
Carbonate is benign - maybe a bit too much so. I wouldn't balk at the lye; it's not as bad as people make it out to be. Don't get it into your eyes and wear gloves. Keep in mind that anything that will effectively soften gelatin will also soften your skin; it's the same principle.

Herzeleid said:

My %10 sodium hydroxide left a clear mark inside the amber glass bottle. May be due to the color of the glass the micro etching more visible in my case.

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Coming back to this as I happened to be messing with a glass bottle just now anyway.
Sometimes, it looks worse than it really is. Take this bottle that has held 75% potassium carbonate solution (Pyrocat HD concentrate B) for a few years:

Note that this doesn't clean out with rinsing, swirling with a piece of cotton/sponge, soap etc. It's easy to mistake this for actual damage to the glass.
After a few minutes' sitting around filled with a dilute citric acid solution, it looks like this:

I didn't bother cleaning it up further at this point; the few remaining traces would have cleared up just fine with some more soaking and swirling.
I did the same with a 10% sodium hydroxide bottle (also used among others for thiourea sepia toning) some time ago, but didn't take any pictures. The phenomenon and cleanup were identical though.

Which isn't to say that carbonate or lye cannot etch glass - just that caking of residue can sometimes look like the result of etching, especially since the caking can be very resilient.

BJC0000 said:

Thanks for the replies chaps.
I'll try bleach first and then carbonate - leaving the lye till last.
Thanks again

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If you're going to use bleach do it outside and spare your lungs.