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EvaV

unglazed stained clay bodies

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Hi everyone, 

Kind of newbie here: I have taken some basic ceramic classes in the past, but now I'm getting back into it (yay). I discovered that I really enjoy working with porcelain, specifically making porcelain jewelry.  

I have a question concerning this matter (sorry if it’s a stupid question – I’m still very much in the learning process)

I want to try and stain porcelain clay bodies (I was thinking about mason stains, though they are quite hard to find here in Belgium). I really like the look of unglazed porcelain, but I was wondering if it’s safe to use the stained porcelain on the skin if it’s unglazed (twice fired (bisque and cone 6), but unglazed). I don’t know whether some of the toxic materials in the stains can transfer to the skin and cause irritation (or other unwanted reactions).

Thank you in advance for clarifying this for me!

Eva

PS: sorry if my English sounds a bit off - I'm not a native speaker. 

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Hi @EvaV

I don't know the answer, but perhaps you could apply a clear glaze to the back of the jewelry, to the side in contact with the skin.

I made some pendants, many years ago, that were not glazed on the back, and they were uncomfortable, so I coated the back with clear nail polish.

And, your English is very good, I understood your question.

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What about using an underglaze, instead of a stain?  If you wanted the color to be more subtle, like a stain, you could thin the underglaze down a bit. 

As @Chilly mentioned, clear glaze might be a good route to take.  You wouldn't need many coats, to make the surface more durable. 

 

 

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hi, eva, welcome to the forum.

look at the website of one of our members who stains clay and creates beautiful work.  her name is Chris Campbell and you can find her in the list of members.  she has not posted much lately but the photos show how much she knows on this subject.   there are previous posts with your exact question as well.

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Stains could be added to the clay body and mixed thoroughly which will require some prep work to minimize the transfer of colorants to all of your working surfaces. Not a big deal, just might want to have cheap/disposable dedicated wedging boards, and working surfaces. Mason stains are essentially encapsulated mixtures of colorants, so they are much safer to handle with your bare skin than raw colorants, however I do not advise spreading them around heedlessly. Likewise, you may not want unintended transfers of colors to other works.

Unless you desire the marbling effect of mixing numerous colorants into your clay body, you may find it easier to apply the stains to the surface of your finished objects. Stains can be mixed 50/50 with frit 3124 and fired to cone 06/04 which will produce durable surfaces which I doubt would have any issues with long term skin contact. The mixtures of stains/frits will settle out very rapidly in a container of water to a very dense cake, which will be a nuisance to re-suspend when needed to apply, so a suspension agent could be added to limit this. As well, it does not have great brushability so agents like CMC gum could be added which will give you an easier time to apply.  

Aside from a pendant worn on the neck, and resting against the chest, I cant think of jewelry objects which regularly are in constant contact with the skin (earrings dangle, broaches/pins are generally on attached to clothes, etc).  So this may not be a big concern in general?

If the frit/stain mixture, or the addition to your clay body still concerns you about contact with human skin, then a sealant could be applied over the finished objects to further protect them. Something as simple as spray on acrylics (come in range of surfaces from matte to high gloss) could do the trick, or an actual glaze, which would be the most durable. You could also dip them in resins, which may provide a different surface than the other two I mentioned.

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I did make coloured porcelain as jewellery years ago.

Initially unglazed I found even with earrings the unglazed porc.  picked up body oils, make up etc and a few people mentioned this to me and so I ended up glazing placing at cool spots , I fired pretty high in those days.

Glazing one side too fiddly for me and suspending on trees at high temp I didnt pursue for long.

Could have made a low fired glaze and put in hot spots in bisque fire I guess.

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45 minutes ago, Babs said:

Initially unglazed I found even with earrings the unglazed porc.  picked up body oils,

Interesting! Were the oils/dirts easily washed away with soap/water, or had they penetrated deeper into the porcelain?

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The solution to the absorption of oils of porcelain jewelry is to burnish the area to be left unglazed, and fire the porcelain until the porcelain is mature! The area can then be polished with fine emery cloth to remove any residual roughness.  The definition of mature is:  zero surface porosity!  

Some potters call this 'vitrified'; others measure water absorption; if the product is expected to resist oil, then measure absorption using oil instead of water.  

LT

 

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+1 for what MMR said.

I made some red clay dishes with a very low absorbency for our home, left part of the clay unglazed on the outside. Before making them I soaked some unglazed test pieces in different liquids to make sure the clay didn't stain. Beet juice, strong tea and canola oil. Left the test pieces in for days then ran them through the dishwasher, zero staining on all the tests. We've been using the dishes for about a year now, daily use, no staining on the unglazed parts. I would expect the same results if porcelain was used.

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On 2/12/2019 at 4:44 PM, Min said:

I made some red clay dishes with a very low absorbency for our home, left part of the clay unglazed on the outside. Before making them I soaked some unglazed test pieces in different liquids to make sure the clay didn't stain. Beet juice, strong tea and canola oil. Left the test pieces in for days then ran them through the dishwasher, zero staining on all the tests. We've been using the dishes for about a year now, daily use, no staining on the unglazed parts. I would expect the same results if porcelain was used.

I found this post because I was researching to see if my own methods would be food-safe. I've been using a cone 6 clay that is labeled as a porcelain (but isn't quite as glassy) that has a 1% absorption rate that I have been "glazing" after trimming while still leather hard with a Mason Stained slip for color and texture. My plan is to only glaze the inside or food surfaces with a zinc-free clear glaze that's also labeled as lead-free and non-toxic, then I'd leave the outside unglazed but finely sanded for a smooth tactile finish. I've been searching to ensure that the pieces will be safe and also durable against stains if I leave the outside unglazed. Any advice  or thoughts further on this topic would be so helpful!

 

Edited by manders

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The outside will be more prone to staining, 1% is significant for such a large exposed surface and glazing only one side of mugs can cause them to shatter suddenly when hot or cold items are poured into them due to the expansion  difference in the glazed and unglazed surfaces.

Just  my opinion though, the test I would love to try is to hand wash the mug, thoroughly dry it and after  a reasonable time to air dry completely, microwave the mug in short bursts to see how much it heats up along the way. If it becomes hot after several 10 second cycles, this would confirm to me it is likely not safe for use in the microwave.

Finally mason stains derive their color in various ways, so it  sounds as if the user will be drinking from surfaces not encapsulated by glaze as well.  Encapsulated stains are thought to be relatively safe as their colorants are encapsulated and will not melt in the glaze. These colorants will now be exposed directly to the user, especially along the outside rim of the cup.

Sorry - Cautiously not a fan of this.

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I would at least glaze part of the outside. Maybe wrap the liner glaze up on the top inch or two, will help keep it from blowing out.  On a lot of my bowls I chatter the bottom half of the outside and only glaze the top half, knock on wood I haven't had soup go flying yet

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7 hours ago, manders said:

I found this post because I was researching to see if my own methods would be food-safe. I've been using a cone 6 clay that is labeled as a porcelain (but isn't quite as glassy) that has a 1% absorption rate that I have been "glazing" after trimming while still leather hard with a Mason Stained slip for color and texture. My plan is to only glaze the inside or food surfaces with a zinc-free clear glaze that's also labeled as lead-free and non-toxic, then I'd leave the outside unglazed but finely sanded for a smooth tactile finish.

Hi Manders and welcome to the forum! Couple thoughts, firstly don't trust the posted absorption rate given by the clay manufacturer, do your own tests with your firing conditions. Secondly, if your Mason stains are coloured with a cadmium inclusion stain then don't sand the pots down. The sanding can break down the structure of the encapsulated stain thereby probably releasing the cadmium. Thirdly, while it does happen that a pot can dunt when only glazed on one side this isn't necessarily always the case. It has to do with how close the expansion of the glaze and the claybody are to each other. To test if your glaze is compatible with your clay make some very thin walled cylinders, about 4" tall. They can just be crudely made but make them very thin walled and put a base on them, I would suggest making 3 or 4 of them. Bisque them as usual then glaze the inside only with as thick a layer of glaze as you can get on there without it cracking when dry. Fire it then place it in the coldest part of your freezer overnight. Put it in the sink and then pour boiling water into it. Obviously be careful while doing this. If it's going to dunt because of a mismatch between clay and glaze it's going to show up with this test. With my everyday individual sized bowls the outside is glazed about 3/4" down from the rim, no issues with them and they are used daily.

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2 hours ago, liambesaw said:

I would at least glaze part of the outside. Maybe wrap the liner glaze up on the top inch or two, will help keep it from blowing out.  On a lot of my bowls I chatter the bottom half of the outside and only glaze the top half, knock on wood I haven't had soup go flying yet

While not super common, much more common in cups than bowls. Glazing 1/2 way down seems to help immensely. It is however completely unpredictable as batches of glaze and clay are not exactly uniform since they contain natural materials which vary.  And of course the natural structural variability of a hand thrown product. I know of no reliable test that will predict this, there are too many variables. Having said that it is not a given and many last and last. When your cup of hot coffee explodes one morning though it will make you cautious from then on. I never sell anything fabricated this way, just out of paranoia I suppose.

Edited by Bill Kielb

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@manders "But isn't quite as glassy"  

from your comment, along with 1% absorption: sounds like they used ball clay as the plasticizer. Titanium content dulls out the shiny- translucent properties associated with porcelain; but the 1% absorption indicates high glass content. I will assume that absorption value came from your own testing?

Several things you can do to ensure success: If you follow the common firing to 2190F with a hold- replace that with a peak of 2230F with a short hold. Secondly, if you have a programmable kiln: from 2050F up to 2230F, ramp at 125F an hour to this peak. The extra heat work will produce a tighter body- and should lower absorption. A cone 6 porcelain body will tolerate heat: you could also fire to a light cone 7.

As Bill pointed out: glazing inside and leaving the outside unglazed causes an unequal stress from the glaze contracting. Getting the COE of your glaze as close as possible to the COE of the clay will be the big factor dealing with this issue. The freezer/ hot water test Min suggested will tell you if you got this right.

if you have access to Laguna Frost porcelain (c5), this might be a better clay option for several reasons. (You can fire it to a light c6). It has premium plasticizers, along with higher body flux content that will produce a much higher degree of translucency. This translates to higher glass content, with zero absorption at c6. The added glass will make the unglazed areas "tactile", to use your description. The added flux will also increase the COE of the clay, making glaze fit easier. Frost has a COE of 6.99- much easier to match. The clay you are using is probably around 5.65. COE ( educated guess from your description)

testing is imperative for functional use.

Tom

 

Edited by glazenerd
Spelling..of course

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On 3/26/2019 at 10:46 PM, manders said:

I found this post because I was researching to see if my own methods would be food-safe. I've been using a cone 6 clay that is labeled as a porcelain (but isn't quite as glassy) that has a 1% absorption rate that I have been "glazing" after trimming while still leather hard with a Mason Stained slip for color and texture. My plan is to only glaze the inside or food surfaces with a zinc-free clear glaze that's also labeled as lead-free and non-toxic, then I'd leave the outside unglazed but finely sanded for a smooth tactile finish. I've been searching to ensure that the pieces will be safe and also durable against stains if I leave the outside unglazed. Any advice  or thoughts further on this topic would be so helpful!

 

Food safe ?  

I take that to mean: food will not be contaminated/poisoned by anything in the clay or glaze, or by nasty stuff that may take up residence through absorption.

Microwave and dish-washer safe are a whole other rabbit hole.

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3 hours ago, Chilly said:

I've been searching to ensure that the pieces will be safe and also durable against stains if I leave the outside unglazed. Any advice  or thoughts further on this topic would be so helpful!

@Chilly

I think there is reference to durability as well

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