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JMWP

Colored terra sigillata turns hazy

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Hello there!

I very much like terra sigillata and have started to make small batches of my own, using the same clay body that's dried and crushed, water, and sodium silicate as the deflocculant. I let it separate, decant the top layer of water and ladle out the sig. I am adding a mason stain (deep brown) after the preparations. I have applied the colored sig to a piece of greenware (a bowl) and a textured bisqued sample. I apply as many coats as needed to get an adequate layer and then polish with a plastic bag and buff some more with a soft cloth. When finished, it's the most beautiful color and sheen I've ever seen! But then as it dries, the color starts to turn hazy and grey. The polish is still there but the color is dull.  I've attached an image of the sample where the edges are starting to dry and turn grey. The center where it's still moist shows how I assumed terra sig should look when dry? As a side note, I am using terra sig in a slightly different manner on the textured sample, kind of like a wash to get variation in color/depth. I really like how the higher areas get polished and the lower areas less so. 

From all the images I have seen with terra sig, all the finishes with color look buttery and bright.  Just curious if this is normal for this color of mason stain or if maybe I'm not doing something correctly along the process. I have added wax to one of the pieces after fired to cone 06 and the wax brought back more of the the color. Also, I tried a sample with plain sig (no added stains) and it's stayed a nice soft sheen. Is there any way to eliminate the haziness? I've tried polishing more as the haze sets in but makes very little difference. Any  suggestions on what I can try or comments would be greatly helpful!  Thank you :)

 

 

 

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Adding stain can affect how a sig burnishes. It may or may not polish up as well as plain sig. Based on your picture, I'd say you don't have enough coverage with the sig to get a really good polish. And any time there's clay showing through, or texture in the clay, the polish won't be as good. Smooth surfaces burnish up the best. Also, if you fire the sig too hot you can lose some shine. It's best to not go all the way up to bisque temps.

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Thanks Neil for your feedback. Much appreciated! I've also covered a  small, smooth bowl with the deep brown terra sig with 4+ layers and polished it. Same outcome, where it is beautiful and rich until it starts drying. Then turns hazy and gray. I've also tried different ratios of the stain to terra sig  with same haziness so I guess it must be the stain or maybe the clay I am using.  It's almost like the thin slip is drying over the stain color., if that makes any sense. I will try out different colors, maybe with ball clay, and test them out to see if I can get beautiful results. 

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8 hours ago, JMWP said:

I've also tried different ratios of the stain to terra sig  with same haziness so I guess it must be the stain or maybe the clay I am using.  It's almost like the thin slip is drying over the stain color., if that makes any sense. I will try out different colors.

If you have other stains available it would be a good idea to run tests with the other colors. Change only one thing at a time so you can nail down the effect of each different variation. If you get the same hazing with the different stains, then go to the ball clay and start over with the brown. Your other option would be to use the ball clay with the brown stain and see what effect you get...if the hazing goes away, you'll know it's your clay. Good luck with your testing

JohnnyK

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Today I tested a black mason stain with the same clay terra sig 2 ways: one on the same textured sample and another on a greenware, smooth vessel. Both polished up nicely but the textured sample hazed a bit, though not as much as the deep brown. Still waiting on the vessel but I can see a some of it turning where it is drying. I put on as many layers as I needed to not see any of the clay underneath but unfortunately there's a hairline crack in the sig where I assume it maybe dried too quickly with as many layers as I put on (too many too quickly?).  

I did try a plain one without stain and at first I thought it was going to be fine but the next day it looked slightly duller. I took a soft cloth and polished it some more which added back a little more shine. I'm firing it tonight to see what happens.

That's an interesting thought about adding wash then the terra sig over it. I will give that a try!

I have to admit, I've only seen this process done on videos so I don't know what is normal when it has dried. I assume hazing is not normal?  Is it supposed to remains as shiny as when you're done polishing like attached image where  I've just finished polishing or does it get slightly duller when completely dried? Most images of terra sig pots have a lovely sheen! If I wax it, there is a lovely sheen, but I'm assuming with terra sig, you shouldn't have to unless you fire it too high. Thanks for all your relies. I will try ball clay next and see what happens.

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Edited by JMWP

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I am using a plastic bag and polishing and sometimes with a soft  microfiber cloth. I've tried a metal spoon and a smooth rock but those seemed to leave marks, which might be because I'm not burnishing properly.  Any other suggestions that I might try?

Today, I tried a bright peacock blue mason stain and a chestnut brown with both the clay body I normally use and a ball clay.  I  got better results with the brighter colors with both terra sig mixes, more like I expected with a satiny sheen when dry with the color still bright. But the black and deep brown both  also turned  shades of gray in varying degrees with the ball clay sig. Maybe it's the dark colors? I wonder how potters get the beautiful black? 

Just curious if anyone might have a picture of a pot with black terra sig on it , but dry, before bisque firing so I can compare to mine? I'm new at this so I have no idea what is normal and what is not.  Maybe black does turn a little gray when it dries?  Or not . . . . :)

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Does anyone know the particle size of mason stains compared to sig? @glazenerd I mostly mean you.:D It takes a lot of stain to color a slip, and I'm assuming that holds true for sig as well. So once you add 15-20% stain to it to get good color, if the particle size is different enough, it could definitely be messing up the polishing. I'm also assuming that stains just don't compact and burnish the same as clay. So then what could be done to solve the problem? Mill the stain down to a smaller particle size?

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If you ball mill many stains you will alter the fired colour of them.  It's also not recommended for any of the encapsulated stains. Greg Payce, who almost exclusively uses sig on his clay work, taught us to add the stains after the ball milling process if you were going to incorporate them. He was big on ball milled sig, and his colourants were mostly oxides because of the breakdown tendencies, but he did use some. I would have to dig through a lot of old papers to find exact recipes. He also wasn't burnishing anything, as he wasn't super interested in a high shine finish on his work, though. The soft surface sig can get even if it looses some sheen in the firing can still be nice.

@JMWP, can you please tell us what clay body you're using? I'm wondering if it's an earthenware that maybe has some kind of soluble in it that's coming to the surface, and it's only apparent because of the dark colour of the stain.

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@neilestrick,  I am not aware of the specific mesh of stain, except to say it is larger. Body stains are the largest, they are used to add color to brick. The rest are either frit or zirconium/silicate encapsulated. So if you use a common frit size: 50-80 microns; or 120 to 200 mesh in pottery speak. In the world of particles: anything over 50 microns is classed as "grain size", in lieu of particle size.

Om4 is the popular terra sig clay of choice; which has a particle distribution of 0.50 to 20 microns. When deflocculated, siphoned, and collected: terra sig averages 0.50 to 5 microns typically. In mesh size- 2500 to 24,000 mesh. So to visualize in our world: mixing sand with golf balls. The whole reason terra sig acts like a glaze is particle size.  So yes, particle size is involved.

however, let me float this observation: I know iron and titanium bond to larger particles and calcium, sodium, and magnesium bond to smaller particles. ( some variations, but mostly  applicable.)  which means terra sig from Om4 would lose most of its iron and titanium content; but its calcium, sodium, and magnesium content would soar. So I tend to believe that a change in clay chemistry is playing in this mix as well. So @Callie Beller Diesel Question about soluble salts is on point as well.

 

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@Callie Beller Diesel I am using Georgie's Trail Mix clay. I have no idea of it's contents but description says plastic and forgiving with sand for strength. Midrange stoneware, off-white after firing.

I recall reading about the particle size of stains being larger and heavier, would using oxides make a difference? If so, what would yield a good black or dark brown?

As a result of my testing additional colors, I found that the peacock and hazelnut brown did very well after firing using my clay body! The colors stayed bright and sheen remained. The black  and deep brown went gray as usual, even with the OM4 clay terra sig.  I did notice various shades of black mason stains so I'll try the one that is called onyx (Co,Cr, Fe, Ni) , which looks very black. I'm using best black (Cr, Fe, Co, Ni), which I find interesting because they both have the same chemical compounds but in different order. Wish I could remember my chemistry days so I can understand why it makes a different shade of black!

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Edited by JMWP

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Well to get a black underglaze recipe folk use mixture of cobalt manganese dioxide and red iron.

Would need to test quantities required but not as safe to handle as the encapsulated stain.

Someone may know percentages.

The action of burnishing may encorporate or expose underlying clay?

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Ok. I went and dug up Greg's black sig recipe.

 His method did involve ball milling and boiling, as he had access to a ball mill as an instructor in an institution and he wasn't as worried about production times. His primary reasoning for using the ball mill was 1) to get the most possible sig out of a recipe, and 2) cobalt oxide can be speckly once fired. If you choose to use the deflocculation and siphon method to make sig and use cobalt oxide, you may get some subtle speckles. I personally find it attractive, but if you are after a smooth flawless colour, it's something to take note of.

80 Water

20 Red Art clay

1 Darvan

1 Black Copper Oxide

2 Cobalt Oxide

1 Manganese Dioxide

 

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10 hours ago, Rae Reich said:

@JMWP, one has more cobalt than iron, the other more iron than cobalt. If it's not going black enough, more cobalt should help.  And manganese  

@Callie Beller Diesel, wouldn't cobalt carb mix better without the grinding?

Maybe. Worth testing. He used oxides because they offer more colourant power. 

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1 hour ago, JMWP said:

@Callie Beller Diesel Thanks for the black sig recipe! I am very intrigued but don't have a ball mill. Though, the speckling is sounding like something I would like to have in the finish.

Any time. Just use the deflocculate-and-siphon method you were using with your other batch to make it. If you don't have Darvan, just use the appropriate amount of the deflocculant you do have. Most studio potters don't own a ball mill unless they're really into refining their own materials. They're loud, and ball milling can take days. Like I said, Greg was trying to get more of the right sized particles for sig out of a single batch, and he happened to have access to the equipment. The siphon method is a lot more practical for most. 

Edited to add: if you're worried about wasting the bottom layer, it can be used as a regular decorating slip. Just don't put it in your reclaim because of the deflocculant.

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