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Light spots on slip coated pieces


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Hi there

Ive been having an issue with some areas of my slip coated pieces getting lighter and hazy when fired. I apply slip with the piece rotating on the wheel using a wide brush and several coats. This is what’s happening on the low bowl shape. The bottom gets kind of lighter hazy scuffed . At first I thought it was from rinsing a bisqued piece and perhaps dust got on the bottom so I never rinsed again but it’s still happening. Any thoughts? 
 

this last firing I have this new lighter area issue on the larger vases. I really don’t know what that could be from. A few guesses are areas that dried more quickly, or areas that got wet when they were unfired. Also could be an issue with the clay body? they didn’t look lighter until they were glaze fired to cone 6.

The stakes are kind of high for these, they’re due soon for a big client and I don’t have time to remake them. 

I don’t normally glaze the outside of these illustrated pieces but I think that would solve the issue. Any tips for clear glazing a piece that’s already been fired to temperature? I don’t have a sprayer but that seems like it may be best? 
 

ive noticed that if I have oil on my hands and I wipe the splotches they darken some. I know it’s very unconventional but has anyone oiled unglazed ceramics? They’re for display only and won’t be used.
 

sorry the slides are oriented wrong, couldn’t get them flipped on my phone. 

 

 

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35877142-B618-4685-B623-4A61AC7AC019.jpeg

B2B729E1-1D06-4B69-BF66-10B75218C203.jpeg

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These are just guesses .... not from personal experience ... I would think that oil will eventually wear off. If they’re just for decorative use, maybe a thin wash of acrylic gloss medium would work? Or acrylic matte medium? Thin it out with water so it’s just a wash. And you'd have to coat the entire piece so the surface texture is the same overall. Once it”s dry, it will be as permanent as anything short of a ceramic solution, Of course, test on something unimportant first!

As for why it happened in the first place, to me the lighter spots look less fluxed. Run the slip through a sieve again, maybe a few times. Or use a stick blender to give it a long whirl. Another possibility is that there is extra thickness in the walls of the pots where the problem is happening. The clay isn’t fully vitrifying due to the extra thickness, and therefore not transferring its fluxing action to the slip. Firing a little hotter might solve this problem too. 

Don’t know if this is a private collector or a gallery, but when I was wholesaling with galleries, there was a time or two when I had to miss a deadline due to a firing problem. Calling them and explaining what happened, and asking for more time always works. These types of buyers understand that you are doing something that isn’t automatic or fully controllable. 

Edited by GEP
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Thanks for the replies. I may try a gloss medium (on a less important piece first). I’m afraid to try to glaze and refire though I may try that too. 
 

still a mystery to me why especially the random splotches would occur. The walls are even thickness and I don’t trim the large vases at all. Almost seems like an issue during firing. It’s only been happening for the last 6 mos or so and nothing in my process or slip mixing has changed. 

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The location of the light spots could also indicate that another pot was loaded very close to it in the kiln, with the pots almost touching each other? Nothing wrong with loading a kiln like that, but it’s another case where firing a little hotter would help. Or firing slower, so the heatwork can reach the places that are more densely packed. 

I agree with Neil that your next step is to use witness cones to measure exactly how hot your kiln is firing, in every zone. There might be a cool zone somewhere. 

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I agree with @GEPthat it looks like proximity to other pieces has caused uneven heat distribution, in the case of the bowl, seems like the proximity was to the shelf. As long as this batch is unglazed, I see no problem with re-firing them more slowly so they all heat evenly. Maybe pack the kiln less densely. 

Great work - love your drawings!

Edited by Rae Reich
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7 hours ago, GEP said:

The location of the light spots could also indicate that another pot was loaded very close to it in the kiln, with the pots almost touching each other? Nothing wrong with loading a kiln like that, but it’s another case where firing a little hotter would help. Or firing slower, so the heatwork can reach the places that are more densely packed. 

I agree with Neil that your next step is to use witness cones to measure exactly how hot your kiln is firing, in every zone. There might be a cool zone somewhere. 

Wow I think you may be on to something. I used to do a 30 minute soak in all my glaze firings. I don’t remember when I heard that would be a good idea but someone scolded me once saying I’m probably over firing. So I stopped. It seems that loosely corresponds to the time this issue started.  It may be that was what I needed to vitrify my pieces.

I always pack very densely. And i never use witness cones since I switched to a computer kiln. Elements are getting pretty old so it’s very possible it’s not getting quite up to temperature. 
 

the slip I use is more like a terra sig. does anyone know if that would have a higher firing temp? 
 

would be wonderful if a simple refire and soak could solve the issue. 

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16 hours ago, dazzlepottery said:

I used to do a 30 minute soak in all my glaze firings. I don’t remember when I heard that would be a good idea but someone scolded me once saying I’m probably over firing. So I stopped.

Aha. My money’s on this being the cause. 

In my experience, a 30 minute soak is about a half-cone of heatwork, which is significant. It’s a perfectly legitimate way to fire, if it makes your pots work. If your pots were not showing signs of overfiring, which I gather they weren’t, then that person gave you bogus advice. And if they delivered it as a “scold,” then that person can mind their own business. 

Instead of testing gloss medium on an unimportant piece, I would put that piece back in the kiln and fire it with the hold. If it turns out your clay/slip combo can withstand a refire, then refire the whole load. If not, then try the gloss medium on this load. 

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On 3/7/2020 at 7:46 AM, oldlady said:

could it be salts coming to the surface?   one of the chemical experts might have an idea.   where are you, Glazenerd?

Sorry AH, not been following the past week or so. Fuming as suggested is one possibility, but more likely soluble salt migration. Nep SY has become the body flux of choice because it is cheap; but also has up to 14% soluble salt (sodium). Over- firing would effect the piece on a broader scale I would think; with blistering and bloating being the more common defects.  As you recall the testing on Little Loafers a year or so ago; soluble salt migration was severe enough to cause glaze defects. Migration in a clay body; tends to collect in the areas that take the longest to dry. From the effected areas, it appears to be following that pattern. 

One question: did this occur in previous batches of clay? Is this a new clay batch, or a change in clay suppliers?

 

T

Edited by glazenerd
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It has been happening the last few rounds of firings. Not sure if it Corresponds to a new batch of clay, it’s possible. This is standards 365 English cone 6porcelain. I’ve been using this claybody for many years. 
 

the worst time was once when I tried rinsing bisqued pieces. I ran the water from the rim to the base and let dry overnight. The pieces weren’t glazed on the outside. The bottom of those pieces were horribly streaked. 
 

also lately when I’m cleaning up clear glaze on the outside of pieces with a wet sponge, those areas tend to have sponge mark streaking. I assumed that was from leaving a residue of glaze that didn’t have enough components to flux. I don’t know much about glaze chemistry but that was my guess. However I’ve never noticed this before and in the last firing I tried hard to use fresh water and wiped many times and still had issues. 
 

Since this thread, I found a photo of the vases before they were bisqued. The irregular Light blotches were present then. I remember areas of the clay drying much more quickly than others. It’s very possible there were drafts when they were drying and these were exposed areas from holes in my plastic, but I’ve not had pieces dry this dramatically in random areas before. See attached, area around dogs leg. I can’t figure out why quickly drying an area would affect it after it’s been fired either. 

CDBE2653-D967-4E0D-8E38-6FE732E9C65E.jpeg

Edited by dazzlepottery
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Are you making your own slip?

Is your pot uniformly moist or dry when you apply slip?

Is your slip just clay and stain?

Do you continously stir slip before loading the brush?

Could the brush be picking up clay from pot?

What temp are you bisquing to?

When you wipe the bisqued pot does your water remain clear?

Is it possible that surface of pot is irregular and so your slip application is of different thicknesses. And in order to cover surface you are inadvertently removing slip over higher areas to get the application on indented areas. Brushing it on instead of laying it on in some areas?

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Yes I make my own slip, out of suspended clay from my throwing water, reduced and mason stains.
 

The pots are leather hard when I apply slip, I let it set up between coats so it doesn’t remove the last layer.

i bisque to cone 06. 

the water often has a layer of glaze in the bottom after wiping the bisque. Perhaps I just need to change water more often. 
 

the coats of slip are very even, as are the walls and surface of the pots. I’ve been making pots this way for almost 10 years, thousands so far, and the problem has only just begun to present itself. 
 

it’s the mystery of the patches on the vases that I’m most concerned with. I can come up with at least some kind of explanation for the streaks from wiping. Still don’t get why a dry patch would still look light after firing. 

 

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You're making your slip out of the clay in your throwing water? That's probably the issue. You have no control over what's actually in that sludge.The silica and feldspar settle to the bottom, and the clay stays suspended, but the ratios in what you're actually using for your slip could be anything. Start making your slip out of actual clay and maybe the problem will go away.

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Actually this last time for most of these teal pieces  I made the slip from clay for the first time. Including that low bowl with the haze. 
 

 I didn’t have enough of my throwing water to mix up the teal. The two vases with the blotches were made the old way ( letting the solids settle and just pouring off the suspended liquid) but the rest were made with crushed clay scraps.

It’s not just a random bucket of wash water, I am careful to only use water from a pure porcelain throwing session, pour into  another bucket to settle further. Then reduce what stays suspended. Just comparing the two slips on these pieces, I prefer my method, because the unglazed surface has more sheen to it. The clay mixed slip is rough and flat. 
 

like I said I’ve made thousands of pots this way with no issue so I don’t think that’s the problem. 

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Standard 365 is a Grolleg porcelain C6 body: Grolleg is as pure as kaolin/clay comes- extremely low levels of titanium, iron, and magnesium: which at higher levels can produce natural color. The MDS states 30% feldspar and 15% Nep Sy ( sodium feldspar): so by actual application- up to 45% feldspar or 45% Nep Sy. (That a lot even for cone 6) 

Soluble salts are carried by the water content in the clay. As clay dries: the water in the center of the wall moves to the surface- which also carries the soluble salts with it. This migration appear as light/whitish areas on the clay surface. In every example you have shown thus far: the whitish areas are towards the bottom, most around the foot ring. The foot ring/bottom portion of a thrown piece is typically the last to dry: making it more susceptible to soluble salt migration. There can be other causes as mentioned; but contaminated slip in my opinion would appear randomly at any portion of the vessel- not just the bottom. Assuming all the vessels in the firing had the same stain and or oxide: that would rule out fuming. Fuming usually results when iron, cobalt, and manganese are fired together: the off gassing fumes from cobalt for example discoloring an iron bearing stain next to it. 

Below is a picture of serious soluble salt migration from natural clay harvested by a forum member (Hi Mary) although extreme, it does give visual context of what occurs when soluble salts migrate to the clay surface.

large.1645359455_HudsonClay.jpg.bd12b8e7ae2be0c144652d843f32a4e8.jpg

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As a workaround until you get this sorted out you might try Spectrum Blue Green 559 underglaze. It very much looks like the same colour as your stained slip. I haven't taken it to ^6 unglazed so I don't know if it fluxes enough to gloss up so you would need to test for that. With my clay I've never seen uneven patches using underglaze like you are having with your slip. Another option would be to make or purchase some underglaze base and colour it with your stain.

edit: I'm just going to add the underglaze base recipe I got from George Vardy at Mason stains. Cut and paste of his email:

One of the best engobe formulas is a very simple one, developed years ago by Ron Mason who then owned Mason Color Works, Inc. Carol Mason, his daughter, now operates MCW.
 
EPK Kaolin 10 parts
Feldspar.     25 parts
Flint.            25 parts
Stain.           40 parts
Mix well with water, add 1 part VeeGum T that has been FULLY broken down in water, screen thro 100 or finer screen, bring to your painting consistency by addition of more water if needed.
When using very strong stains such as cobalt blues or chrome greens you may need to reduce the amount of stain if the fired color is too strong for your requirements. 
This can be used to decorate on clay or bisque. Be sure to dry fully before glazing.

IMG_3143.jpeg.4a2e317d57b042be57c7862704352c34.jpeg

Edited by Min
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I would just go with a commercial underglaze, or figure out a slip recipe that you like that can be accurately reproduced. The problem here is that we don't know what exactly is in that slip you're using. It's probably mostly kaolin, with some portion of neph sye and feldspar, and some portion of solubles. But it's not going to be the same every time, especially the solubles. And if Standard makes a small change to one of their ingredients that isn't noticeable in the thrown pots, it could still show up in your process. You're kind of halfway to making a terra sig, but you're not deflocculating. A sig might be a good solution for you, though, because it could provide a surface that has a slight sheen.

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7 hours ago, dazzlepottery said:

 

i bisque to cone 06. 

the water often has a layer of glaze in the bottom after wiping the bisque. Perhaps I just need to change water more often. 
 

 

Not sure what you mean by glaze if you are wiping bisqued pots.

Can you explain?.

Doesn't matter how carefully you collect your throwing stuff tje ratio of whatever is in it may be quite different.

Worth changing method as Neil suggests even uf only to see if this is your prob.

What water are you using.

Recent bushfires changed ph of water here in local reservoirs.

Use distilled water....

Just throwing you thoughts

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right, couldn't understand the glaze if only wiping pot in prep.  TO glaze.

Thanks for explaining.

You are not inadvertently getting palm or hand marks during that process or the carving process..unseen until fired ,which would come up after bisque and glaze firing? ....fine porcelain dust.. damp hands pot getting laid on ..

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Guess eliminate one by one would be the way.

Uniformity of slip

Cleanliness of hands and bench before and during every process.

How you get rid of crusty edges when carving...

Where are you placing brush ..onto finely dustladen surface...

And so on....

Good luck.

Could video yself at work...may get an aha moment.

Try drying pots on rim....

Or try drying in rods on foot...

One dried in draught..coincidences always stuff you up in life.. 

One dry in closet

Whatever.

Good luck.

Not bored are you?

Edited by Babs
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