Encapsulated Compounds

[JoC]

I have a query about sanding and firing.  I use mid fire porcelain which fires to 1200C.  I use body stains to colour the liquid clay.  I sand the pieces to a very smooth finish after the bisque fire.  If I have understood the chemistry, the firing provides encapsulation of the stain compounds.  I think the sanding breaks that encapsulation on the surface being sanded.

If that is so, does it correct itself in the final firing?  If I don't do any further sanding will the final firing encapsulate the stain

compounds again?  This temperature takes the piece to vitrification. 

 

I'd be very grateful for some advice here.  I am exploring food safety in my work.  

 

Thanks, 

Jo C.

[glazenerd]

Encapsulated stains have a different mechanism: other than what you are describing. The manufacturer guards the process tightly: so the end user is only guessing at the mechanism. Encapsulated stains usually include selenium, cadmium, lead, vanadium: and otherwise harmful elements. These elements are highly toxic on their own: so I strongly advise you stop sanding immediately. Secondly, the usual zirconium encapsulating method does have a peak temperature: where it does dissolve, and these elements are released into the glaze. That temperature is usually 1220C; however the right mixture of fluxes, or the use of powerful fluxes such as lithium carbonate can lower that temperature. You are currently firing to 1200C: so you are right at the doorstep of releasing these elements into the glaze matrix: where it can then leach into any fluids put into a bowl, cup, or plate. I HIGHLY recommend you do not use these encapsulated stains on any pottery pieces intended for functional use.

I use selenium, cadmium, and vanadium when experimenting with various glaze experiments. But NONE of those pieces ever leave my shop, and are plainly marked. I have the safety equipment to deal with them. I also follow the EPA guidelines in disposing of them, as well as storing them. I keep very small amounts around: and they are locked up where no one can have access. I am not trying to dissuade you from using them: only making it very plain the potential hazards to you and your customers. 

 

Nerd

[neilestrick]

I've been reading a lot lately about these red, orange and yellow stains that will go to cone 6-10, and there just doesn't seem to be any convincing evidence that they're definitely safe. I've found a lot of statements that say they're safe IF.....way too many variables to be definite about any of it. In general, though, if they're only being used on the outside of a pot you should be safe. Just keep them out of contact of mouths or food/drinks. In a studio like mine, with many students using many glazes, there's no way I would put them into use. There's just no way to guarantee that someone won't use them on the inside of a pot.

 

In testing a couple of the inclusion pigments, I will say that the color isn't that great. Because the stain doesn't go into the melt like normal stains, the glaze colors don't have the depth and intensity that I expected. You have to use a lot of stain to get halfway good color, which makes for a very expensive glaze.

[Guest JBaymore]

William Carty has repeatedly said they are safe as far as cadmium release goes.  Highly stable.  

 

Please remember folks... cadmium (and other stuff) is potentially in far more items than GLAZES that you might use.  Red, orange and other such bright colors of UNDERGLAZES and OVERGLAZES and underglaze pens and markers and such can have cadmium (and other "nasties") in them also.

 

If your work has lead or cadmium compounds on it....... anywhere....... you are bound with the US FDA and the State of California laws.

 

best,

 

.............john

[Min]

A few years ago I contacted Spectrum, Crysanthos, Mayco, Mason and a few other suppliers/manufactures of underglazes, glazes and stains. I asked about the breakdown of the encapsulated stain if the product was sprayed. From the replies that I got I was told that ball milling would break down the stain and it was strongly advised not to do this. For use in the spray gun none of the companies advised against it and also said sieving was fine. I'm thinking sanding the product is going to be somewhat akin to ball milling. Even if you had a few pieces lab tested for leaching it would not rule other untested pots in as being safe since the sanding process is not going to be the same on every piece. It would be interesting to see lab results on pots that have been sanded, though I wouldn't want to be the one sanding the pots. I wonder if Chris Campbell has any insight into this?

[JoC]

Thankyou all for such quick responses.  It is  a contentious issue.  After all the research I have done I will only be using the cadmium encap. stains for non functional ware and 

on outside surfaces.   I am extremely cautious when handling, masks, gloves and I sand with wet/dry paper in a bucket of water, outside.  It is a very small part of my work.

I don't use it in a glaze, I use it in the clay body which is why I am wondering whether the final firing captures it.  

It sounds like there is no definitive answer to this one, I have exhausted my enquiring, I have spoken to professional potters, ceramic retailers, manufacturers and chemical engineers dealing

with food safety testing on commercial ware. There are different opinions right across the board.

I have sent pieces for testing to the US to try and get a measure at least of what might be happening with my process.  I think it is probably also good practice to use a glaze on the inside of the

ware with these particular stains.  To achieve the shapes I produce I cannot use a glaze so these colours might be better out of the repertoire.

There are plenty of colours out there to use which don't have this measure of concern about them.  

I use Cool Ice and Southern Ice Porcelain and the mason stain colours come up beautifully when fired.  

Thankyou all again.

[mugmkr]

Hi Jo, there is an excellent glossary entry on encapsulated stains at this link: https://digitalfire.com/4sight/glossary/glossary_encapsulated_stains.html

Sounds like the manufacturing of the stains are very toxic to the environment, only India and China have lax enough environmental laws to permit their manufacture. Not to mention as others have said these do not produce very interesting glazes. Do you really want such highly toxic compounds in your studio? And how do you know that the pot you made with non functional purpose won't, at some time, be used for food? Once the pot leaves your studio, how it is used is out of your control. If you are making sculpture, no worry. Otherwise you might be surprised at how your pot gets used by your customers. Lots of great colors available from safer colorants that have stood the test of time. So it's not as though you are stuck with only cadmium stains to provide color in your glazes...my two cents for what it's worth. Cheers from snowy Central Oregon! Owen

[JoC]

Hi Owen, thanks for taking time to pass on your thoughts.  After much exploration and listening to the various viewpoints regarding the cadmium I won't be using it in anything  to sell.

I completely agree that you have no control over how someone will use your work.  

It is interesting that it is still available to be purchased.  Why is it not in the same category as lead?  While I am happy not to use it myself I continue to be interested that it is available and

I would still like to know what happens to it embedded in the clay body (i.e. not on the surface as a glaze) at vitrification.  This is how I use all the mason stains, in the clay body.

I have read many times and been told that vitrification encloses the colour.  Is doing a final sand to give that beautiful silky feel that porcelain gets breaking that colour seal?  I haven't quite

had that one answered.  I get that it would at the bisque stage because it isn't properly vitrified.  

It is very fascinating none the less.  The colours when used in the white clay body are beautiful.  The cool ice porcelain is a great product to work with.

I understand Chris Campbell uses that clay and the mason colours so I might email her and see what her take is on the sanding process.  

Thanks again for your time.

[Chris Campbell]

First, I use mostly Mason stains and have only tried a couple encapsulated ones over time. I am very happy with the colors I get from using Mason stains.

Multiple Warnings on packages tends to discourage me because my chemistry background is extremely basic.

 

Important note is that my work is non-functional.

If I make a bowl shape I generally put holes /slits in it so it can never be used for liquids ... only dry food.

When I sand I wear protective breathing gear and work over a bucket of water so most of the dust falls into it and settles to an easily disposable layer.

 

My recent experiments with purposeful color placement in thrown work was done with Mason stains since the vessels are functional. Look for an article on this method coming soon in Pottery Making Illustrated.

[oldlady]

chris, it is a little late but i just saw your article in the 2013 issue of ceramics monthly.  i only get to see it occasionally and was glad to catch your article re: coloring clay.   thanks, it is great that your website gives so much info on the subject.