Playing with Mica
First, all mica is not the same. Then there is the slip that is used as a carrier. All slips are not the same. So if you are wanting to work with mica on the pots, some experimentation will need to take place.
Our slip is very "plastic" and needs to be applied to a green pot, an unfired one. So some amount of planning is required. The slip will simply fall off of a pot if the slip was applied to a bisqued pot. This is mildly irritating as we tend to make pots in the winter and store them in bisque form for finishing in better weather. All of our kiln work is done out-of-doors.
We could reformulate the slip but what we do now is part of our system. If it ain't broke, we don't try to fix it.
To the slip we add interference mica. Not any other kind. We've tried dozens of micas but find that only the interference mica will work. It's about the heat. Additionally, of the seven (or so) colors available of the interference mica, three work excellently, one works "okay" and the others, well not so much. The mica can be mixed and use in combination. The slip can be layered or used in a sort of umbra between color The slip can be brushed or sprayed. We do any and all of that.
We have used the mica with the slip. It works but it fragile until the spray finished is applied. Washing the pot to remove the excess carbon also take the mica away. But the mica is more brilliant used directly and we will use it in that manner for some pieces.
The only caution is not to put the slip on too heavily. Well, our slip doesn't want to go on too heavily. Yours might be okay with a slathering.
The amount of mica needed is about three teaspoons to a half pint of slip.
Remember that I wrote that all slip is not the same. We're talking terra sigillata when we talk slip.
A hint, if you are brushing on the terra sig, you may find that the slip in your mica mix becomes thicker over time from the clay being brushed from the pot and added back into the mica mix. That hasn't been too much of a problem, and when it get a bit sludgy, I add more water.
A lot of this work is "by eye." Experience matters.
I bisque all my work at cone 04. I know you will read that terra sig is best fired to ^08-^10 and so forth. I'm happy for other's experiences. I just don't want to play that game. If this process hadn't worked at a cone 04 bisque, I'd walk away and still live. I'm just too old, grumpyand maybe a bit lazy to keep track of bisque temps. ALL my work is bisqued to 04.
I use an electric kiln for bisque and propane fired "clam shell" raku kiln for the mica work. An electric kiln will work for the mica, I am just set up with the raku kiln and it is easier for me to switch back and forth between techniques using that one kiln for the center of my world.
I find that the temperature to watch is the a range between about 900 and 1300 degrees F. It's that range that carbon will "stick" (below 900 it's really just black goo on the surface) and when it will continue to burn and burn off the pot (above 1300). Ideally you want to be between those temps. You can get some pretty cool halos of color with less carbon above 1300, so that's worth playing with. Below 900 there isn't much penetration of the carbon.
All that said, it is the smoke one is really trying to create. The carbon traces from feathers or hair or sugar or molasses is really creating the smoke which, when it passes over the mica infused terra sig, reveals the colors. So play with smoke.
Holding the pots at different angles will give different motion effects with the smoke direction. Trying this on a windy day will leave a small amount of color against the carbon trace of the hair (for example), but the blush of color normally sought with this technique may be effected by the wind.
I use feathers and horsehair with a smattering of sugar. I've used molasses and find it best on larger pieces. In my hands the molasses was pretty crude.
If you want to blow you mind. Apply this mica concoction on a pot and use the obvara technique over it. Opal comes to mind whenever I used this combination.