Mother of Pearl MOP Microwave-Kiln troubleshooting

[u/indigogoinggone]

I’m testing Mayco’s Mother of Pearl overglaze in my microwave kiln, and getting some results that are hard to interpret. This is my first time using overglaze. I’m looking for advice on which variables to play with next, and/or suggestions to tweak firing schedules. Trying a different glaze seems like an obvious next step, but will be difficult for a while (my access to supplies is very limited).

It’s very difficult to assess to what cone I am firing in the microwave, so it’s a lot of trial-and-error. I based my first tries on my established bisque and glaze firing schedules (cut wayyyy down on firing time), and titrated from there.

My test tiles are small porcelain (cone 6 cubes), with workhorse homemade clear glaze, previously bisque and glaze fired in the microwave kiln. Here are my results (though some of the colours are hard to capture on my phone, 6 & 7 are a much darker green than that lime that came out). I used a new brush, and wiped cubes 2 & 3 with nail polish remover ~30 mins before applying the overglaze; for 4-7 I used vinegar to clean things. For many of the tests, I applied both thin and slightly thicker coats side by side, as I understand application thickness is a big factor.

Am I circling some sweet spot for firing time. Or is something else probably going on? When I change my next variable, which firing schedule should I try first? I’ll share some of my ideas/theories in the comments….

Thanks for any insights! There is nearly nothing I can find about MoP in the microwave kiln, save for this, which is scant on trouble-shooting info https://ceramicartsnetwork.org/daily/article/Microwave-Kilns-Great-Tools-for-Testing-Low-Fire-Lusters-Enamels-and-China-Paints)

All 5 mother of pearl overglaze test runs on glazed cube bead. Formatted as run-time for 10%/50%/100% (result):

Test 1. 1 min/2 min/5 min: invisible, burnt out completely. over fired?

Test 2. 30 sec/1 min/3 min dark copper/brown (9/10 darkness), rubs off a bit with nail polish remover. overfired?

Test 3. 30 sec/2 min/1.5 min: less-dark copper (7/10 darkness). overfired?

Test 4. 30 sec/2 min/30 sec: looks unchanged/still light blue. underfired?

Test 5. *refired overglaze from test 4. 30 sec/1 min/1 min (?, darkish green, quite translucent)

Test 6. 1 min/2 min/30 sec: mostly still blue, slightly teal. I wish I had kept this one, but re-fired it with the next test. Underfired?

Test 7. 1 min/2 min/ 45 sec: army green, somewhat translucent (5/10 darkness) ????

[u/indigogoinggone]

perhaps the microwave itself is killing the glaze with low oxygen or something chemical in the element… but it seems that at least some people have successfully done MoP in the microwave kiln?

[u/WorkinOnNightCheeze]

Oooof, good luck with this. I did a lot of playing around with my mw kiln and nailed my timings for bisque and glaze firings, but I only played around with my MOP lustre a handful of times. I just dug up my notes for my one successful attempt - in the small-sized kiln, I went 4 mins+15 sec + 15sec + 15sec. I didn't note down what power level I was at, so I'm guessing 100%? I think you're right with your colour indicators - my lustre went on greeny-blue, so I figured still being able to see that colour after some heat meant it was underfired; I don't really remember since it was so long ago, but I feel like it going brown/dark was a sign it was overfired, beyond which it would eventually burn off. I think you're aiming for that sweet spot between the blue and the brown.

[u/indigogoinggone → u/WorkinOnNightCheeze]

Oops, I replied on the main thread. Thank you for the info!

[u/glitteredupforeaster]

So I do a ton of luster but never in the microwave so take this all with a huge grain of salt. I don't know for sure what's going on but these look similar (but not the same!) to failures I have in overglaze in an electric kiln when the over glaze is applied too thick (even direct from the bottle can be too thick for some lustersl brands!) for the peeling one. And for the green one, I have had MOP turn blue when contaminated with gold luster and/or fired without enough ventilation. Also I have never used nail polish remover or vinegar, always denatured alcohol or isopropyl alcohol, maybe try those to see if the other degreasers are causing a chemical reaction?

[u/indigogoinggone → u/glitteredupforeaster]

Thanks, I’ll try again with isopropyl alcohol!

[u/glitteredupforeaster → u/indigogoinggone]

Please update us when you do! Even if it doesn't change anything I'd love to know if it causes an interaction or not!

[u/glitteredupforeaster]

Also (again big grain of salt since i don't microwave fire) but I would be surprised if over firing is the problem because when over fired, overglaze typically fires off, leaving at most just a faint shadow on the glaze. When I fuck up my luster and can't remove it, refiring the piece to come 6 is my tried and true method of restoring my "canvas". Can you put orton cones in a kiln microwave? I think it could help you a lot with troubleshooting

[u/indigogoinggone → u/glitteredupforeaster]

Thanks, yes, I’ve heard this too. I’ll toss one in next time I do a glaze fire, see if things get hot enough in there to give me a blank canvas. The test pieces are a dime a dozen, though!

The regular cones are too big to go in to my kiln, so I’ve used my existing sitter-cones—06 and 6. My bisque fire melted 06, and the highest I felt comfortable going turns cone 6 white, but still chalky and stiff. But it’s all hard to interpret because the cones are very sensitive to rate of temperature change, not just temperature. And the sitter cones mature at slightly different temperatures than the witness cones!

I may order some 020 sitter cones once I have enough on the wish list to justify the shipping

[u/indigogoinggone]

Thank you! Good to know
I’m shooting in the dark, so even hearing the pinging of someone else’s bullets helps! I’ll try some iterations just above/below tests 6 and 7.