Chilly

Unglazed work is not a problem in any way, it has to do with what you want it to look like. Chilly mentioned that earthenware and bisque are basically identical temperature ranges. If you’re new to ceramics, understand that people can be technical without realizing they’re speaking a different language. All parties walk away thinking they communicated when they did not. Bisque essentially is earthenware, and would require no separate firing, so I’m wondering if by “earthenware” you mean something else. You could be thinking of unglazed stoneware. 
I think this comes down to the clay you’re using. Clays are rated by temperature (and other ways, such as plasticity and shrinkage). The three common ranges are cone 06-04, cone 5-7, and cone 9-11. Clay undergoes a change in its character when fired to maturity (its cone rating). 

Earthenware and bisque are so close to the same temperature, one firing will (should) be sufficient.  
At the centre where I volunteer, we fire both at the same time.
If you want it higher fired, it can be done in one firing, but must be very (very) dry.
 

Earthenware and bisque are so close to the same temperature, one firing will (should) be sufficient.  
At the centre where I volunteer, we fire both at the same time.
If you want it higher fired, it can be done in one firing, but must be very (very) dry.
 

None that belong to me.
Robins nesting under the worktops on top of the cupboard. 
Found a long-tail til dead on the floor yesterday.  Probably came in through the open window/door and couldn't find it's way out.  Suspect it had flown at the closed (glass) door and knocked itself out.
Lots of bees and butterflies come in through the open window and either get rescued by me or expire in the summer heat..

Still arguing for using the same body for mug and sculptures. If you want/need to attach hand-build sculptures, why not hand-build the mug.
101 on making a slab mug, with tips on rolling out the clay if you are unaccustomed to hand-building
https://thepotterywheel.com/slab-built-mugs/
Personally I prefer to  join round a tube of some sort, as in
https://clairepalmerphotography.wordpress.com/2014/05/13/how-to-create-a-cylinder-vase-slab-building/

This part stood out to me. Just because they come from the same manufacturer, that doesn’t mean it’s the same clay, or that they’re compatible. They may have similar shrinkage rates, but check to see if they have the same firing range. If it’s not common to mix them, ask why.
If you are excited about slipcasting and incorporating sculpted parts, I would suggest making yourself a flat plaster bat, and pour a layer of casting slip on that to create a slab. You can then use the slab to build hollow parts with, similar to the method Peter linked to. Or there are a LOT of slab building tutorials that could be modified. Look especially for online classes or workshops that mention soft slab techniques, as you will have to work efficiently. Cast slabs can soften quite a bit as you work with them. 
If the casting slip slab is the same thickness as your other slipcast pieces, you can attach them fairly easily. This will work best to create relatively lightweight, hollow pieces. 

We charge by weight at the centre.  Plus a little extra for some glazes, or a little less if only once fired.  

I'm with Bill. Making people calculate volume is a big pain for everyone involved. Asking most people to do that sort of math is not a good idea. Weight is a much easier way to do it, and you can easily have different pricing categories for big wide pieces like serving bowls that take up a lot of volume without much mass, and cups and such that are more compact. Or better yet, just include glazing and firing costs in the price of the clay. It keeps students from feeling like they're getting nickel-and-dimed to death, takes away a cumbersome step in the process, and makes it easier for people to  track and budget their pottery class spending.  I've done it by weight from day one, and it works fine. It's easy enough to figure out the approximate weight of clay in a typical load and calculate all the associated costs of firing and come up with a price. Plus it motivates people to throw thinner with less trimming, and makes them think more about the technical aspects of clay work like avoiding cracks and warping and whatnot that may keep the piece from getting fired.

I have physical projects and virtual projects.
I can have anything between 1 and 10 physical pieces of clay on the go at any time.
Virtual projects live in the privacy of my own head, and I don't know how to count them, but there are lots.  Most are totally unachievable with my skillset, but I love to look at something, and think I could make that with a differnt shape here, or a different colour there or ....

I have physical projects and virtual projects.
I can have anything between 1 and 10 physical pieces of clay on the go at any time.
Virtual projects live in the privacy of my own head, and I don't know how to count them, but there are lots.  Most are totally unachievable with my skillset, but I love to look at something, and think I could make that with a differnt shape here, or a different colour there or ....

I have physical projects and virtual projects.
I can have anything between 1 and 10 physical pieces of clay on the go at any time.
Virtual projects live in the privacy of my own head, and I don't know how to count them, but there are lots.  Most are totally unachievable with my skillset, but I love to look at something, and think I could make that with a differnt shape here, or a different colour there or ....

None that belong to me.
Robins nesting under the worktops on top of the cupboard. 
Found a long-tail til dead on the floor yesterday.  Probably came in through the open window/door and couldn't find it's way out.  Suspect it had flown at the closed (glass) door and knocked itself out.
Lots of bees and butterflies come in through the open window and either get rescued by me or expire in the summer heat..

I have physical projects and virtual projects.
I can have anything between 1 and 10 physical pieces of clay on the go at any time.
Virtual projects live in the privacy of my own head, and I don't know how to count them, but there are lots.  Most are totally unachievable with my skillset, but I love to look at something, and think I could make that with a differnt shape here, or a different colour there or ....

Hey Pres, I got a good one from my wonderful partner:
How many pieces do you work on at a time during your normal workflow? 

as a small-time amateur, it's good to hear that even the pros need to do this
 

I never look at that view, but if it bothers people there are other ways of viewing posts....
I click on "unread content" and browse from there.  I sometimes click on browse and click the big button beside equipment, to mark them as read, as most of that category goes over my head.

@Mark C. I decided to bisque again to not risk losing the artwork and I’m glad I did too. I read what you wrote and decided to not take the risk. 

I have seen matte questions a bit here so ……
Just noticed this came out! So posting it here in the event you are still working on these. Sue does a great job of explaining and providing answers to the question: How do I make a matte glaze / more glossy (Stull matte anyway) ? You might find it worth the read https://suemcleodceramics.com/how-to-turn-a-matte-glaze-glossy-with-one-ingredient/
She does mention other mattes and also provides good insight on under fired mattes etc…. IMO

Just one little picky thing---please don't teach kids--or anyone, really-- that they are "painting" on clay. They are using glaze, not paint; they are glazing, not painting. There is a difference--a big difference--and it doesn't serve them well to leave the experience without knowing the difference and learning correct terminology. 
 

Oh well, here's another try.
Dear Wise potters, thanks! You were right: after tweaking the engobe recipe, using local whitish clay as slip, slowing the bisque firing and also slowing the glaze firing now my pots have only some very small pinholes.
That involved some protracted procrastination, while I indulged in other techniques, and then 'forgot' to add cobalt to the glaze. That's now done. 
Three cheers! and thanks again.

 
 

Hi Roberta, I believe the thicker the better yes, probably I am using too little glaze, maybe combined with the fact that I did not use cones so I can not confirm temperature it gives me an idea of both the problems 
Thanks!

Hi Chilly, thank you for your help, I did not use cones, I will next time to ensure they are not underfired.  thank you

Try pouring the slip out more slowly.  If it fills the pouring hole, it will cause a vacuum and pull the lining with it.
There was similar questions asked before, with  other suggestions.  I'll see if I can find them.

Pick something really easy, like a piece of fruit - apple, orange,  banana -  or an existing cup or mug.  (Bear in mind that anything cast will end up smaller than the original.)  That will get you thinking of undercuts and how to section off.  

Are you using cones to verify the temperature of your kiln?  My guess is they are underfired.