I will defer to the BE experts on this because there's probably a better way...but when I need transparency I have far more control over the glass color and bubbles by using a custom billet.
Make a refractory mold using a container that holds about 20% more than the volume of your model. (I used to use deli containers, then I had a set of well-drafted "mixing molds" made out of refractory) Choose a reservoir/crucible for your glass that you can prop as high up in the kiln and above the mold as possible. The idea is that the glass mixes and loses bubbles on the way down, so the more height, the better.
Set it up in the kiln and if you're using frit, block the reservoir openings with a couple of pieces of sheet glass to keep it from falling into the mold when it's cold (or you can prefuse the frit to a sheet of glass, just depends on how much work you want to do). Then start layering in your glass. The configuration of the gate (hole(s) from the reservoir), shape of the mold and the placement of glass dictate the color patterns in the billet.
You have to experiment to figure out how the glass will flow, but it usually pulls down from the center bottom, up through the center and across the top, and then from the sides. You can take advantage of this by stacking the reservoir horizontally--seems counter-intuitive, but you get a LOT more mixing action.
Then fire--I leave it at process temps sometimes for several hours to fine out the bubbles. I go for a fast anneal (I don't care if it breaks), take it out and see if I like the mix. If not, I break up the billet and refire it the same way. It'll also mix as it goes into the real mold, so keep that in mind.
This method opens up all sorts of possibilities if you start thinking about reservoirs, gates and the shape of your model. You can get some pretty stunning color blends and placements with it.