Chilly

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.

Orton — auto-correct once again. 

Thank you for all your replies! This is my original idea that I've landed on (so many ideas...  ), but I'm open to suggestions  . I am planning a combo of wheel thrown (the center "stem", which goes all the way to the top to become the center of the flower) and hand built (the flower and leaves). In the center there will be a place for a bolt (there will be a hole in the glass) and then secured by the ball on top, as well as glued to the glass. Look forward to your feedback!
**Edit: forgot to mention there will be 4 of these legs

Some eye-candy and tips on slip-cast multi-layer work - but for bone china rather than parian ware.
Multi-Colored Casting in Tempting Transparency
https://ceramicartsnetwork.org/pottery-making-illustrated/pottery-making-illustrated-article/Tempting-Transparency#
Some examples of Sasha Wardell's  work at http://www.strathearn-gallery.com/artists/435/sasha--wardell
PS Can you use setters with parian ware? It might be useful if you could.

Sorry I probably didn't explain it the best. I don't mean paint a layer and let it set and then do the mold.
Once I have the mold box set up and ready to cast in plaster I will weigh out my plaster and cold water in the right ratio. Add the plaster to the water and let it sit for 1-2 min then gently mix with my hand for 2-3 minutes until I feel it slightly thicken. Now that I am ready to pour the plaster into the box I will use a brush dipped into the plaster and paint onto the form to get into all the detail and then pour the plaster into the box to cast the form. I found if I don't brush the plaster onto the form and just pour the plaster into the box I end up with bubbles on the casting surface.
 
I got the idea from this video, he is not using a box to cast the form and lets the plaster thicken up after brushing a lot more than I would to pour the mold but the brushing idea is the same. Later on in the video when he is pouring the plaster head you can see the kind of consistency I would pour the mold at after brushing it onto the form to capture all the detail.
 

There's no problem using a plasticine form to cast plaster molds from, it could be a bit tricky to block off areas for multi-part molds but some normal clay will probably remove pretty well when you come to pour the next section.
I did read some plasticine can mess with silicone curing but nothing about plaster.  My only advice is to get a soft natural hair brush and paint on a thin layer before you do the main pour, that's the only way I found to get rid of most/all air bubbles on the surface. Tried different surfactants and different soaking times and mixing times and they all seemed to trap bubbles on the surface no matter what.

He has issues with auto-correct 

A 30 minute soak is quite long so I would think that would be more heat work than 1250 and no hold but they are probably quite similar. I would stick with the 1250c, not that they say if it was at 60c per hour for the last 100c of firing or 150c per hour for the last 100c of firing which also would make a difference. 
 
Looking at the running on their picture I would add some 'cookies' under the pots as you haven't tested the glazes before.

that photo should be preserved as the best example of black coring photos.   any way to insert it into one of the resources mentioned for future searchers?

@oldlady, I will add a few tags to this post so it’ll show up in search.

Hi Claire and welcome to the forum!
Yes you could use slabs for a sink that size.  4 - 6 mm thick slabs should be fine. I would miter the corners so there is more surface area for the joins.  I know there are many broad firing range clays used in the UK, I would suggest using one that you can fire to the top temperature to get a strong mature body.

Yep. The only issue is that it may not fit properly, but that's true of all clay bodies. What fits on one may not fit on another. As always, run a test before you use it on something you care about.

Received this from Amaco after inquiry. Easy breezy to roll on thickened underglaze (after drying somewhat), as several mentioned here on the forum. Easy to clean up, too. Good tip on attaching plastic wrap on freshly printed slab when constructing to avoid smearing.
https://www.amaco.com/clay_how_tos/314
 

Thanks, I'll try that.   
Thank you so much.  That solved the problem.  I learned something new!

Hi and welcome!
If I’m understanding you correctly, you want to make a positive to make your mould’s negative around. You can do that with just about anything. I was taught to do it with waster clay (clay you were going to throw out anyways). But you can use things like modelling clay, spray foam insulation, dental or non dental algitek, or cardboard. Professional slipcasters make a master (a positive used to make more copies of the mould) out of more plaster. That last one is for folks more confident than I am. 
If you’re using clay, you’re probably only making one mould with the resulting master. The heat from the plaster is enough to make clay dry enough to shrink, but if you only need one, it’s usually the cheapest and most available option. You’ll still want to coat both your silicone baking pan and the resulting master with some kind of mould release. Mould soap is the professional go to, but if you are dabbling/experimenting with your first moulds, dish soap (Sunlight, not Dawn) diluted by half, or Murphy’s Oil Soap are commonly used substitutions. If you’re using any kind of oil based mould release, you’ll have to remove the residue with a solvent so it doesn’t interfere with the mould’s absorption, but soap can be wiped away once the mould has cured.

Personally I doubt it's practical, but this at least tells you not to overheat it (see the last sentence in the quote).
Might be worth a try in a domestic oven.
https://en.wikipedia.org/wiki/Plaster
Chemistry
See also: Calcium sulfate § Hydration and dehydration reactions Gypsum plaster, gypsum powder, or plaster of Paris, is produced by heating gypsum to about 120–180 °C (248–356 °F) in a kiln:[18][13]
  CaSO 4 ⋅ 2 H 2 O ⟶ heat CaSO 4 ⋅ 1 2 H 2 O + 1 1 2 H 2 O ↑ {\displaystyle {\ce {CaSO4.2H2O {\overset {heat}{{}->{}}}{CaSO4.1/2H2O}+ 1\!1/2 H2O ^}}} (released as steam). Plaster of Paris has a remarkable property of setting into a hard mass on wetting with water.
CaSO4⋅12H2O+112H2O⟶CaSO4⋅2H2O{\displaystyle {\ce {CaSO4.1/2H2O + 1 1/2H2O -> CaSO4.2H2O}}} Plaster of Paris is stored in moisture-proof containers, because the presence of moisture can cause slow setting of plaster of Paris by bringing about its hydration, which will make it useless after some time.[19]
When the dry plaster powder is mixed with water, it rehydrates over time into gypsum. The setting of plaster slurry starts about 10 minutes after mixing and is complete in about 45 minutes. The setting of plaster of Paris is accompanied by a slight expansion of volume. It is used in making casts for statues, toys, and more.[19] The initial matrix consists mostly of orthorhombic crystals: the kinetic product. Over the next 72 hours, the rhombic crystals give way to an interlocking mass of monoclinic crystal needles, and the plaster increases in hardness and strength.[20] If plaster or gypsum is heated to between 130 °C (266 °F) and 180 °C (350°F), hemihydrate is formed, which will also re-form as gypsum if mixed with water.[21][22]
On heating to 180 °C (350°F), the nearly water-free form, called γ-anhydrite (CaSO4·nH2O where n = 0 to 0.05) is produced. γ-Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants. On heating above 250 °C (480°F), the completely anhydrous form called β-anhydrite or dead burned plaster is formed.[19][22]
 

Glaze firing was a success!!! I think I should make a few more like these and not concerned about any air trapped like everybody said!
Thanks everyone for your time, tips and comments !!! 
Greetings from Greece !!!

Just thought I'd throw these out there. ^6 Oxidation faux celadon on ^6 porcelain.   Selsor Faux Celedon ^6 Oxidation (hold for 20 minutes) Whiting 18.5 Neph Syen 25.8 EPK 18.8 Silica 31.1 Gerstley B 4.6 98.8 Copper carb 0.3 Mason #6600 Black 0.02 

Trying something new... I have always worked on the wheel and not done much with slabs or molds. Recently, I purchased a 10"x5" hydrostone butler tray slump mold. I am thinking of texture design work on the inside flat surface of the tray.  My plan is to roll out the clay on cloth, put the mold on top, flip it, let the clay sink into the mold, pull off the cloth, do my design work, let it dry slowly until I can flip it out of the mold.  Am I on the right track?  I would appreciate any helpful hints...
 

elaine, sounds fine to me.   i have found that when i carve things into the face up area that sometimes some sharp bits are left from the edge of the cutting tool.   i use a stylus for most of the work but to eliminate the possibility of leaving something sharp i  lay a finely woven curtain fabric  on top and slide my thumb over the whole piece.   adding slight pressure flattens those sharp bits and lets me see if something needs more attention.   the fabric i use does not transfer thread marks to the work and prevents obvious thumb prints.
have fun with slabs, they do amazing things because you can catch them at just the right moisture level to do about anything you can think of.

I dont think it would burn because of the lack of oxygen. I thought soaking the skewer with a flammable liquid like alcohol or petrol and then set it on fire but I didnt do it because even then it probably wouldn't burn where the clay was round the skewer.. well it's in the kiln cooling down at the moment. It's about 220°C so in about 8 hrs I'll know if cracked.. 

Ok guys my pieces survived the kiln with no cracks and no exploding for sure!!! So happy I got them out safely!!! Glaze time now :)