High Bridge Pottery

Thank you for the video, i think I may have watched this one before! It's a great video but never thought to apply the technique to what I am doing now, makes sense though. I'm just afraid to take the plunge because it could easily be a three-piece mold because of the undercuts and detail. But actually after watching this I may take a break and do something more straightforward and cast that first just to get my feet wet. Thanks for the imput!

I would say in general colouring the clay and glazing with a transparent gloss is going to be the safer option for the end user but there's so many variables in pottery that there's no one size fits all answer. 
 
https://digitalfire.com/picture/L9hivqD172
On this page Tony says "Stains are inherently much safer to use than raw metal oxide colorants because they are sintered as colorant/stabilizer blends. And much less is needed." So it is possible to make a black/coloured glaze that's food safe but it requires testing.

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.

I’ve never done it personally, but I’ve seen tutorials about painting parts (or all) of the plaster mould with a layer of coloured slip and letting it set up a little before pouring the “main” casting slip. It allows you to just use a coloured version of the slipcasting body of your choice to decorate, and it’s thoroughly integrated into the piece. 

So there’s a few points here I think. First, safety, and next practicality. All of the below assumes you have your heart set on a black and white marbling combo at cone 6 (1200 C) in an electric kiln. 
Safety:Most of the risks involved in pottery are to the potter, not the end user. Your exposure to any of those materials is going to be greater than anyone who’s using the end product, because the materials aren’t bonded to anything the way they are in fired ceramic.
That’s not to say that there are no risks, or that you don’t have to mind your chemistry so you have a stable glaze. But your safety is the most at risk in this scenario. Worry a little about your customer, but worry more about your own health.
Because the example is Manganese, most of that oxide’s dangers come from inhalation. Humans only absorb about 5% of the manganese they swallow, and it’s not absorbed through the skin. (Source cited here). So if you are not working next to a kiln full of manganese glazed wares while it’s firing, and you’re using good housekeeping practices that reduce airborne dry clay that uses manganese as a colourant, your chances of injury are lowered. That said, if you’re firing in reduction, using iron in any form as a colourant would be a much safer option if you don’t mind a very dark brown/warm black colour (disclaimer to come). If you’re firing in oxidation, a stain is safer than manganese.
Stains are designed to be more stable/less harmful than certain colouring oxides, and they create colours that are harder to get with oxides alone. They’re not soluble in their unfired form, so you don’t absorb them through your skin or anything like that. Some still contain spinel, so breathing the dust is still bad for your lungs, but only in the same way dry clay is. 
From a practical standpoint, If you’re making marbled clay by adding a colourant to a white clay body, this is the best method to ensure pieces don’t crack apart along the marbling lines. So what are the pros and cons of manganese, iron and black stain?
Iron and manganese are both pretty inexpensive. Iron is a lot safer to use for the potter than manganese, but doesn’t give a true black by itself. Both however, will off gas a lot more than a stain will. Black clay has been very popular the last few years, but all of them that use manganese or umber are prone to bloating if fired to cone 6 (roughly 1200 C). Manganese dioxide begins to break down into manganese oxide at 1080 C. This turns it into a more active flux, leading to bloating. The release of extra oxygen bubbles all creates a lot of pinholing or glaze blistering that is pretty difficult to resolve in an opaque glaze, and will make a clear glaze cloudy with micro bubbles. The few clays that I’ve seen that use iron or iron containing minerals don’t bloat any more than a normal red clay body would if overfired. But you’ll still have issues if you’re using a clear glaze because of that clouding. You can get a clear glaze to go *mostly* clear over an iron bearing clay, but it’s a lot of work and testing.
If you use a black mason stain in particular, they’re pretty refractory and don’t have any Loss On Ignition values, so that means they’re not loosing gasses. Solves all of those above problems, but they do cost a bit more. 
 
 

I feel it is similar to the way we are making metal oxides safer by forming them into a glass with silica and alumina but the stain is already doing some of that for us. I am unsure if in melting the glaze it then breaks down whatever crystal matrix they formed in the stain and releases metal oxides into the melt, I assume that is more temperature related and the hotter you go the more metal oxide is release and no different to using raw oxides except in the % of the recipe as stains seem to need less for the same colour.
 
There is a bit on his stains page that says "These processes render them more resistant to dissolving in glaze melts, or melting themselves, compared to the metal oxides from which they are made." https://digitalfire.com/glossary/ceramic+stain

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.

I would say in general colouring the clay and glazing with a transparent gloss is going to be the safer option for the end user but there's so many variables in pottery that there's no one size fits all answer. 
 
https://digitalfire.com/picture/L9hivqD172
On this page Tony says "Stains are inherently much safer to use than raw metal oxide colorants because they are sintered as colorant/stabilizer blends. And much less is needed." So it is possible to make a black/coloured glaze that's food safe but it requires testing.

Still looking for Orion work; lost a lot of info when my old laptop took a dirt nap. Brownell did similar studies as well. 

You will be much better off painting on the coloured slip when the clay is leather hard. If you paint it onto bisque it will probably crack and fall off, I tried this recently and you can see from the results that painting onto leather hard clay was much more successful that onto bisque. There is probably some way to get it to work but it is much easier to paint onto leather hard as they are both shrinking at the same rate.
 
You might get better results so probably worth testing as parian porcelain is much more glaze like than my clay. Looking at a few recipes there's only 20% kaolin clay in the parian instead of 60% ball clay in my slip.  
 

 
 

I wasn't trying to claim it is safe to fire the kiln or good of the electrician to miss the inconsistencies just that a 30 amp socket/plug on a 40 amp breaker fits the code.

@High Bridge Pottery, re temps and what is happening in the bisque firing, do you have a copy of  Hamer's The Dictionary of Materials and Techniques? If you do have a look at "Firing", it goes through all the stages and temps etc, really thorough read. The figures mentioned by Hamer mesh with what is in the pdf by Steve Davis below. (also the bit from GlazeNerd above re inorganic carbon burn out range) Includes organic carbons and sulphur etc also.
https://www.aardvarkclay.com/pdf/technical/Bisquefiring.pdf
@steviepeas, if you are making functional work that is to be used with liquids, it is best to use a clay with less than approx 1.5% absorption. With wide firing range clays, like so many are in the UK, it's best to fire them to maturity then check the absorption. If a clay is fired to less than the top maturity temperature / cone then it's apt to weep moisture if not. 

Edward Orion, Jr. (yes, the cone guy) did the early studies on inorganic burnout back in 1906-1910 period. In reading his abstract: he cites inorganic burnout between 1250 to 1750F.
T

I'm not so convinced that 800c is really really low for carbon burnout. There doesn't seem to be a real consensus around temperature. Thickness, time,  air flow and the amount of carbon present all play their part. You could bisque to 1000c with rubbish air flow and thick pieces quickly and get less burnout that a slow bisque with good air flow to 800c.
 
Digital fire seems to say anywhere from 370c-950c others say that naturally occurring carbon in clay burns off at higher temperatures: up to 1200 F to 1400 F (650 to 760c) so I think temperature is just one variable. 

I also just say I’ve learned so much about kilns in the past week on this thread! Way more than I learned in my college ceramics overview of kilns haha

I'm not so convinced that 800c is really really low for carbon burnout. There doesn't seem to be a real consensus around temperature. Thickness, time,  air flow and the amount of carbon present all play their part. You could bisque to 1000c with rubbish air flow and thick pieces quickly and get less burnout that a slow bisque with good air flow to 800c.
 
Digital fire seems to say anywhere from 370c-950c others say that naturally occurring carbon in clay burns off at higher temperatures: up to 1200 F to 1400 F (650 to 760c) so I think temperature is just one variable. 

There are electromagnetic fields (or something like that) coming off the kiln, and they get worse as the kiln gets hotter. The two kilns I've had to add a ground wire to didn't have any problems until about 1900F. I remember talking with an engineer from Skutt once and he said the bricks make all sort of electrical noise as they get hotter.

Seems less of an investment buying new elements than buying a gas kiln. 

Thanks for posting the link. My hunch is this comes down to expectations and the history of how glazing and firing methods evolve. We in western cultures seem to prefer "flawless" surfaces; pinhole and dimple free yet in other cultures pinholes and dimples add beauty to the pot. Reading some of the old clayart posts by Lee Love (who lives in Japan) mentions bisque firing to ^012 or cooler. Doing this is going to make a much more fragile pot, I would imagine glaze tongs could very well break some pots. Porcelain bodies have much less junk to burn off, you can get away with a cooler bisque for those bodies than say a high manganese body if your goal is to have a pinhole/dimple free pot. I believe it was Ron Roy who was one of the champions for raising bisque firings to ^04, many people have had success with this. I remember when I was learning about firing the norm was to bisque to ^08 - 06. 

According to the Paragon web site, they should be 8 ohms each. Something goofy is going on with the one reading 5, the one reading 9.2 is worn out, and the others are close to worn out. On the kiln serial plate, what is the max temp, and what is the amperage draw? Does it have a 4 prong plug?