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I am interested in upcycling second-hand crockery by reglazing it, for example the porcelain plates, bowls, and cups that are abundant in second-hand stores. I need a way to remove the outer glaze to re-expose the porous surface of the underlying fired clay so that fresh glaze can be applied

I've read about people using physical methods to remove fired glaze: sandpaper, grinders, sandblasting (eg. here)

The idea of using strong acids or bases to remove fired glaze is suggested in a Ceramic Arts Daily forum post here, with no clear answer about whether it would work.

Sodium hydroxide is a readily-available strong base that slowly dissolves silica glass (the main component of most ceramic glazes). According to this post on Chemistry Stack Exchange a 1 mol/L concentration of NaOH at 100˚C will remove ~1 micron of borosilicate glass over 1 hour.

I'm unable to find an exact number for the usual thickness of glaze on porcelain crockery but I'd estimate that it tends to be in the hundreds of microns.

Given appropriate concentration, time and temperature, would using sodium hydroxide to remove fired glaze allow me to prepare porcelain crockery for reglazing?

Henry Howard
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    Possible , but very dangerous. I worked in a foundry where silica sand traces were removed from stainless castings. Most employees would not even go in the room despite filter mask, hood ,and robe. – blacksmith37 Oct 04 '23 at 16:03

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It doesn't sound very feasible, for a few reasons.

1 Mol/l is quite a concentrated solution of NaOH, and will do nasty things to your skin, then you want to boil it as well, so it will splash. I've worked in chemistry labs, so I would do it, but I'd be very careful indeed around choosing suitable gloves and other skin covering (lab coats are good because they protect your arms but can be removed quickly if something soaks through them). Here's some safety information from the US CDC.

Then you still only get etch rates of 1µm/hour, and a layer which I estimate (and a quick search agrees) to be in the high tens of microns. So to strip the layer, you'd need to boil, maintaining the concentration against evaporation and reaction, for a couple of days in total.

The etch rate will increase rapidly at higher concentrations, and at 100°C you can get to about a 9 molar solution. The risks go up as well. And you need a reasonable volume to maintain the concentration and allow movement.

You also need to consider the end of the process. Glaze thickness isn't constant, and most glazed ceramics have unglazed regions (e.g. a ring on the bottom of a cup). What does a concentrated NaOH solution do to unglazed ceramic. Cosmetic damage to an unglazed out-of-sight bit is one thing, but when you've etched the glaze off the most visible areas, there will still be some left where it went on thicker (sticking with a cup, at the join to the handle, and quite possible in the bottom). I'd expect some uneven etching of the unglazed surface, but that's a guess.

If there's a pattern under the glaze, this will have soaked into the clay to some extent, so won't be (completely) removed with the glaze. That may or may not matter.

If you really want to go down this route, it might just barely be viable as a finishing step after mechanically removing the majority of the glaze, depending on whether the NaOH solution attacks the ceramic itself.

Chris H
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  • Amateur soapmakers seem to routinely use 30% w/w solutions of NaOH which I believe corresponds to 7.50 mol/L at room temp, and they boil it. The risks seem on-par with soapmaking – Henry Howard Oct 05 '23 at 11:18
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    Fair enough, but in their case it reacts out pretty quickly (getting off to a good start within minutes). And everything I read says that you don't boil it - the routine process only heats to around 40-50°C, though dissolving solid NaOH releases enough heat to get hotter than that. I'd say boiling for tens of hours is riskier – Chris H Oct 05 '23 at 12:27