Help Diagnose my PC Problem - Flaking on Hammer Test

[rototerrier]

I decided to change up my routine a little and coated up a small batch and just ran into an issue I've not had before.

Normally I tumble in my RCBS with black ASBB's for 5 minutes and get a fairly even coverage. I bake in a convection oven on 400 for 20 minutes. It's the big nice over sized digital Oster model.

I wanted to see just how much coating I could get without going so far that it puddled during the bake.

I tumbled for 20 minutes. It coated them very thick, but still not so much that I got puddling.

I then baked on 425 for 20 minutes. I probably shouldn't have bumped up the temp, but I did so thinking the coating was so much thicker that it might need a little extra heat.

My normal 5 minute tumble and 20 minute @400 bake yields great results and they pass the hammer test. I can smash them from all angles into a little cube and no cracking or flaking.

This batch, well you can see from the photo, didn't work out. I smacked it a couple times and the coating shattered.

Assuming I didn't over coat them, since they look amazing and they didn't puddle....

Question: Did I over bake or under bake? I'm thinking I over baked / overheated them. What flaking/cracking an indicator of?

[rototerrier]

Little bit of research on my own indicates I got it too hot. Would appear there's no such thing as baking too long as long as it's not too hot. Guess I'll be sure to keep it at exactly 400 from now on. Glad I only did 100...but that's why it's called testing.

[Smoke4320]

Yes too thick and too hot will give that result
It a combination thing

[garym1a2]

When I back too hot I would get paint sticking to screen. Best to cookto the profile of the paint supply. They know their pant.

[OS OK]

The same happened to me last week doing some clear gloss...when it came out it looked like a shade of light grey...? WTH? When I checked the temp dial I noticed that it had been changed to higher temp, just ever so slightly. I used a paint pencil to mark the dial with a thin line and it had been bumped (?) just the width of the line to the higher temp side.
Did the hammer thing and it flaked off...shot them anyway and when I looked at them they were flaked too in the rifling marks. They didn't lead, but I didn't want this so I changed her back.
Hate these little ovens with an analog/mechanical dial...one of these days I'm going to spend the bucks to get a higher quality digital oven that's a little more trustable and larger to boot!

[dilly]

I got an oven thermometer for mine and I have noticed the temperature setting is really crazy. - tiny twist of the knob takes you from 350 to lead melting temperatures.