Wellllll since no one has answered yet and I have absolutely no Idea, Spearmint on scraps. ;~) IIRC Amononia displaces moisture and if it does not hurt the wood it should not harm the hardboard IMHO.
I do know that some tempered hardboard is made with a tannin based adhesive to reduce formaldehyde emission, and guess what ammonia reacts with ...
Based on what little of my organic chemistry I remember, I suspect that any reaction with the ammonia may just make the hardboard harder/brittle, but I don't have time to do the testing and was hoping for a chemist in residence, or "...a yeah I tried that, don't do it".
Damn ... I just hate to do $omething for someone that is going to bite me in the butt two years down the road.
Ammonia will ruin shellac (you can use it as a stripper for old shellac). However it does nothing special to hardboard and even tannates will only be darkened by it.
More conjecture I fear, but based on a bit of experience.
Tempered hardboard seems to have an affinity for absorbing moisture if it's not sealed. Learned this via my TS cast iron top which I cover when not in use with a piece of HB. Surface rust within days even through TopCote. Sealed the HB with a wash coat of shellac and the problem went away.
Ammonia fumes (NH3) and water (moisture) combine to form ammonium hydroxide (NH4OH) which is, depending on concentration, a pretty powerful base. If it won't eat cellulose fiber it's the only strong base I've ever heard of that won't. I used a nail board made from HB to support some cherry I fumed and it was real spongy after 48 hours in the fumes - I suspect moisture in the HB, and then good old ammonium hydroxide had its way with the fiber.
Suggest you seal the HB with a coat of varnish, poly, anything but shellac before fuming.
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