Jeff Longtin

Clay and Glazes for the Potter was required reading when I was in college.   We even had to write some essays about the subjects we read about.    I still have my worn and tattered book.   Denice

Great story Mark. 
Coincidentally I brought in a copy of "Clay and Glazes  for the Potter", by Daniel Rhodes, this morning, to introduce a younger employee to the types of books I referenced in my younger days. I asked her if she was familiar with "Alfred" as being the center of the universe, in the pottery world, for many years. She nodded that she heard of it but wasn't familiar with  it beyond that.
It certainly had an impact, on this young potter, from afar. 

Another option, though hugely tedious, is to keep the mold moving, after you have filled it with slip. That way the bubbles are kept moving and can't settle in the plate wall.
You don't need to do it for the full 30 minute set period, but a few minutes, to move all the air bubbles, will usually suffice.  Filling the mold only half way, during this period, is the most effective. Once you've swirled the mold a few times then fill it to the top.
Years ago I delivered a few molds to a customer who used a "roto-caster" to cast large cement table bases. By keeping the mold in rotation they were able to cast large cement pieces with thin walls. I realized it might also work with casting slip and air bubbles. It does. 

Is it safe to assume the piece is about 30 cm wide? That's a large diameter for a slip cast slab of clay.
I would suggest a smaller form within the piece supporting the top. Ideally the bottom surface, of the top slab, would be unglazed. 
I do this on several pieces. I apply kiln wash to the top of the inner support and the clay piece comes off easily.
A bit of a pain but it almost entirely eliminates slumping of wide, and flat, slip cast forms.
ps. after the inner support form has been high fired, cone 6, I keep it on the shelf and re-use it when necessary. The kiln wash prevents it from sticking to the piece of the kiln shelf.

Welcome to the Forum NotaBot.
There are several reasons why a glaze might "pop". Not so much the glaze itself but something "other" thats gotten into the glaze. Perhaps while you were glazing you set the unglazed pot on a dirty shelf such that something then got into your glaze when you dipped the pot? Perhaps there is a contaminate in your clay that exploded when it was heated? Perhaps you applied several coats of glaze and they had an unexpected reaction to each other? 
I've been making cone 6 porcelain pots for 30 years and rarely had this happen. (Mostly single dip.) These last few years I've been working in a pottery shop where we use cone 6 stoneware glazed with multiple layers of glaze. We loose elements regularly due to glazes popping. 
 

And here are the final photos of the mini 'Hadrian' custom glazed equine ceramic to a rich carmel dark palomino with satin glaze.  I think his owner will be very pleased with how well this commission came out.  She picks him up this weekend!  Thanks for following along and I hope it wasn't too boring.  I'll probably play with these final photos a bit more to brighten them up since they are a touch dark.
 

For those with a similar question I'll share this: soda ash is a very strong deflocculant.  It is rarely used on its own and commonly paired with sodium silicate. ("N brand" as I know it.)
From what I've read the sodium silicate works on the easy to defloccuate particles while the soda ash works on the harder to deflocculate particles. (As there aren't as many of those, in a casting body, you don't need as much.)
Darvan 7 and 811 really came on the scene, for potters anyway, in the 80's after the publication of the book, "Moldmaking for Ceramics", by Donald Frith.  (A wonderful book for those just starting to make molds.) Until the book came out most casters used the sodium silicate/soda ash combo to deflocculate clay.  (Also adding barium carbonate to iron containing casting bodies, to reduce scumming.)
The other reason soda ash is only used in small proportions is that it is very caustic. Caustic to humans and caustic to plaster molds.
 
 

Sometimes I combine mold parts to test out new design ideas.  Sometimes the mold parts do not fit well together. I find filling the small holes/gaps with slip, FROM THE OUTSIDE, to be  a simple remedy. Let the slip set up for a few minutes and you have a "sealed" mold.  THEN fill the mold cavity with slip.
You can also fill the gaps from the inside but I find it hard to get my hands inside smaller molds.
 
 

I work at a pottery shop where we have a few RAM press machines. I make several air-release molds each day. (I go thru a lot of Ceramical.) 
Your situation seems odd? If you're getting air bubbles, with no clay, it should be able to express the tile out. Can you post a picture of the mold and the tile form? 
Are you getting air over the entire surface? Could it be that when you made the mold the air gate moved or wasn't held it place securely?
When you mixed the Ceramical did you follow the instructions? USG suggests water at 77 degrees. At one point I used really cold water and that affected the purging.
Are you using old Ceramical? All bags are dated. What is the date of yours? 
 

Being a slip caster myself I sometimes run into this.  From my experience, 30+ years, its mostly a process problem.  (A picture of your mold would help.) Placing a conical cup form into the casting, right after demolding, often reduces this problem. You can eyeball round but having an actual round cone form makes it much easier. Similar to a comment I made yesterday you can also minimize warping if you remove the piece from the mold and THEN cut the rim. My cup forms come out of the mold with the sprew still in place. I place the clay form on a banding wheel and cut the sprew with an exacto knife. I then place the cone form back into the cup and allow it to set up for awhile.  

I was introduced to Seeley and Bell Porcelain slip back in the 90's. (When I made a few molds for doll artists.) I found both to be very white and very vitreous. I also noticed that both developed green mold fairly quickly. Recently I've returned to mixing my own and found that recipes that use English Grolleg Kaolin have the same qualities. (Whereas recipes with EPK or #6 Tile do not.) A recipe from claybucket.com is proving to work nicely. 
10,000 gms water
80 gms Darvan
100 gms Bentonite
11000 English Grolleg Kaolin
4600 Nep Sye
4400 Flint