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White Spots On Bisque-Fired Brown Clay


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#1 Tenyoh

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Posted 02 June 2014 - 09:45 PM

Hi ceramic artists,

 

What causes white spots on bisque-fired brown stoneware clay? Will they disappear when I fire the piece to cone 5? (I initially intended to not glaze or stain it.)

 

I asked you about single-firing a sculpture about a week ago. Your answers were so helpful. I truly appreciated it. However, the ceramic studio did not allow me to single-fire my sculpture because the studio policy does not allow anyone to fire his/her piece alone. I just bisque-fired my work to cone 05. It might have been a good thing, as I noticed bothersome white areas here and there on the piece (the first photo below). Will they disappear when I fire it to cone 5? (It has to be fired with other studio participants' glazed pieces.)

 

I thought the white areas might be caused by hard water we have here. However, thrown pieces hardly have those. Some pieces have lots (like the cat in the second photo), and some have none (like the coiled one).

 

If you do not think white spots will disappear with mid-firing, I'm thinking of staining the sculpture with red iron oxide and then firing it to cone 5.

 

Thank you very much.

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#2 Benzine

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Posted 02 June 2014 - 10:07 PM

It doesn't by any chance wipe off with water does it?

Also, the sculpture still looks great. You can breath a little easier, when handling it now.
"Anything worth believing, is worth questioning"

#3 Tenyoh

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Posted 02 June 2014 - 11:27 PM

Benzine, you're always quick to respond. Thank you very much.

 

No, the white spots did not get washed or wiped off with water.

 

Yes, I feel relieved that the sculpture, so far, has no cracks.



#4 High Bridge Pottery

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Posted 03 June 2014 - 02:45 AM

That has happened to my bisque before where it seems to have gone a little hotter in places and 'over bisqued'

 

I think it will even out but I glazed over my work so not 100%


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#5 Isculpt

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Posted 03 June 2014 - 03:42 AM

Beautiful, creative sculpture!  I've had the same problem on red earthenware pieces that I didn't intend to glaze, but was then forced to color with underglaze to hide the white spots.  One option, if you don't want to fire it again, is to touch up that area with watered-down acrylic craft paints.  It's a slow process and difficult to match the color of the fired clay, which is why watering down the paint is a necessity.  On occasion, I have had to color the entire piece with watery acrylics that still let much of the clay color show through, in order to get a consistent color. (See image of woman holding bird)  Another option, shown on the "Brazilian" bust, is to use a watered down ivory colored acrylic paint to dry brush the piece, hiding the over-bisqued areas and adding texture and depth while leaving the natural clay color exposed.  (On both of these pieces a second firing was done at 06 after I added a copper carbonate wash to create the black shaded areas.  That firing was done before I added the acrylic washes.)

 

I haven't tried firing the pieces to cone 5, but I'd be surprised if that resolved the issue.  I think High Bridge is right; the area is over bisqued, and it's not likely to revert to a uniform color.  But please let us know the outcome if you do fire it to cone 5.

 

jayne

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#6 JBaymore

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Posted 03 June 2014 - 07:15 AM

It is called "scumming". It is because there are some soluble salts in the clay body. They are migrating to the surface as the water leaves the clay and depositing there. On edges where the water tends to get air currents and evaporate first and best....... it tends to get concentrated.

 

Left unglazed it often still shows. Covered with glaze, it is a "crap shoot" as to how it will affect a particular glaze.

 

The FIX is done in the clay body formulation. If you are buying commercial clay...... you are kinda' stuck with that happening now and again. The typuical one is a TINY additiona of barium carbonate to the body (yes... barium carbonate). Thru ion exchange in the wet clay the barium carbonate becom,es insoluble barium sulphate ....and .... problem solved. Getting the precise amount of barium carb. is tricky. You don't want too much or too little. And it is a TINY TINY TINY amount.

 

best,

 

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


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#7 clay lover

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Posted 03 June 2014 - 07:35 AM

I had that look recently on some red stone green ware that was large and needed to dry slowly.  I had to be out of town during the first of the drying time, so I covered it with plastic for 3 days, then changed that to cloth when I returned and could monitor they drying.  The white did not brush off and was still there after bisqing.  I have not glazed the pieces yet  Other pieces from the same bag of red ^6 clay that were smaller and dried quicker did not have that look.



#8 High Bridge Pottery

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Posted 03 June 2014 - 08:27 AM

So has the clay slightly fluxed with the extra salts? This has only happened to me once which seems strange as all my pots are dried the same way. I thought it was to do with my placement of work during the firing.


                                                                                                                 1384226_215924051918490_1181728069_n.jpg


#9 Tenyoh

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Posted 03 June 2014 - 10:47 AM

Thank you all for the responses. I did not know that soluble salts in the clay, or contamination during the clay manufacturing process, was the cause. For now, I would hate to experiment with barium carbonate. I will have to settle with other solutions.

 

Jayne, I loved your work and your finishing technique. You have certainly passed the "inexperienced" stage of clay-sculpting. I will think over what technique to use in order to hide the salt deposit, but I will probably experiment with watered-down acrylic paint. I like the matt effect of the "woman holding a bird." You must have really watered down the paint.

 

Thank you again.



#10 mregecko

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Posted 03 June 2014 - 01:01 PM

I have this happen sometimes with darker clay bodies. As was said above, it's usually scumming; but it can also be dust / detritus loose in the kiln from pieces from a white body, or even kiln wash if you're unlucky.

 

I'd say about 80% of the time, I can fix it with some vigorous sanding in the bisque stage. Obviously take proper precautions to not breath the dust from sanding.

 

If this doesn't work, then your best bet is to cover it up. It can absolutely show in the final firing.



#11 Tenyoh

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Posted 03 June 2014 - 01:41 PM

Thank you, mregecho. It seems that the white spots on bisque-ware may be difficult to avoid as long as I use public facilities. I have no control over kiln maintenance and clay contamination can easily happen there.

 

I will try to sand off the white spots today and go from there.



#12 neilestrick

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Posted 03 June 2014 - 02:22 PM

I have hd the opposite experience, where the scumming rarely shows once the piece is glazed.

 

FYI, if anyone ever needs to add barium carb in their clay mixing process, it needs to be well blunged with water before being added to the clay mix. Otherwise it won't disperse and will leave you with little white speckles of barium.


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#13 shelaghpeterson@aol.com

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Posted 03 June 2014 - 07:28 PM

I think you may be right about the hard water being part of the cause of the spotting.  I say this because the kitty, which looks like it may have been made by a younger artist has much more spotting, especially where parts have been joined.  I think children have a tendency to use much more water.  As for the thrown pieces not displaying the spotting, most thrown pieces have had their surfaces ribbed off and/or trimmed which would remove any surface water.  If your studio water is very hard you might try using water from another source.



#14 Tenyoh

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Posted 03 June 2014 - 09:44 PM

Thank you for all the comments and advices. Today I spent more than 2 hours sanding off the scumming. As it was quite significant I wondered if the soluble salts are not only from the clay manufacturing process but also from the cellulose fiber that I use in my paper clay. I am going to contact the clay manufacturer and see if they can recommend any brown clay that is the least likely to cause scumming. I may also haul the tap water from my home to see if it makes any difference.



#15 Min

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Posted 03 June 2014 - 10:03 PM

I don't know if this is an idea that would work but just throwing out a thought here....

 

Would washing the piece with an acid like vinegar or muriatic after it is fired to maturity then sealing it when dry with a masonry sealant work? My logic is that this is like the efflorescence that happens on brickwork so a similar treatment should work? (big question mark!)



#16 oldlady

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Posted 03 June 2014 - 10:06 PM

seeing all these beautiful sculptures reminds me that age 12 I wanted to be a sculptor when I grew up.  too bad, but it is not too late.

 

last year I decided that when I grow up I want to be Betty White.


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#17 Pres

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Posted 04 June 2014 - 10:17 AM

I have had this happen in areas where I would work to much smoothing out an area with water and tools, maybe the water was hard(tap). I don't know, I do know that this often happens in some clay bodies and not in others. John is right on about the solutions to it, and I can back it up with research I had done years ago about the same thing. Thing is, I am reluctant to keep barium in the studio, so I have to look for other solutions. As far as showing up after glazing. Never noticed it after the glaze firing.


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#18 Roberta12

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Posted 04 June 2014 - 05:11 PM

I have had that happen with buff colored clay, did not know what it was (I do now!) but it was covered with glaze.  It appears that Jayne did a fabulous job of coverage with washes.



#19 Isculpt

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Posted 05 June 2014 - 08:54 PM

Thanks, Roberta, for the compliment.  My mentor taught me that there is no such thing as a ruined sculpture and I have taken that as a challenge, devising lots of ways to fix my mistakes! 

 

Tenyoh, the reason that the woman holding the bird has such a matte finish is that I used a wet sea sponge and when the piece was leather hard, dabbed her face, neck and hands so that it had a velvet-like "nap" to the surface.  I've only used that technique once, but I really liked the effect.  It's nearly impossible to do on a small, detailed sculpture, though. That piece wasn't large (maybe 12"), but it didn't have lots of nooks and crannies to try to get the sponge into.  Using watery washes works best on a piece that hasn't been fired to vitrification. 

 

I have hard water, but I have a salt-based water-softener unit, which may have something to do with the white spots.  It seems somewhat logical that applying water softened with salt could cause a similar effect to "scumming".  I wonder ......

 

jayne



#20 Benzine

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Posted 05 June 2014 - 09:33 PM

Jayne,

 

After growing up with a Father, who sells water softners, I know a thing or two.

 

You may be onto something with that theory.  The softened water has sodium in it, which replaces the "minerals" that make the water hard.  But it's just the sodium, not salt, so I don't know if said sodium alone would still cause scumming.  

 

I have a softner, of course I do, and I've never had a scumming issue with any wares I've made at home.  Then again, I've never had any issues with scumming, in any of my classrooms either, and I can guarantee you that none of their water is soft.


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