Pitting Issue!

[oceancity]

Hi all, 

 

I've been having an issue with pitting. I don't have control over the firings because the owner of the studio I go to does all the firing.

 

I've just started more thoroughly cleaning the bisque ware before I glaze, but I haven't gotten those results out of the kiln yet. 

 

Is there anything else I can do other than cleaning the bisque ware before glazing? Keep in mind I have no control over the firings.

 

More info...

When i glaze I almost always get little pits in the raw glaze that I then smooth over with my finger. 

 

When trimming I use a rubber rib to smooth everything so there aren't usually rough spots and the clay body is a cone 6-10 white stoneware that is pretty smooth.

 

I also sand my greenware before the bisque firing and I'm wondering if the dust left of the pots after sanding could be contributing?

 

Thanks!

 

[Min]

Hi ADL,

 

Couple things kinda jump out at me from what you wrote. Hope you don't mind some questions...

- Cone 6 - 10 clay, are you making functional pots with this clay? 

- Do you know what the firing schedules are for both bisque and glaze?

- Is it just one glaze or several that have pitting? Any recipes available?

- Do other peoples pots fired in the same kiln have pitting also?

- Could you post a picture of the pitting?

- Is it pinholes in the glaze that you smooth over with your finger? This is normal if so.

 

I don't think it's dust on the pots causing the problem, dust usually causes crawling of the glaze. Also, it's a really bad habit to get into sanding greenware, could you use a damp sponge instead to "sand" the greenware with?

[oceancity]

Yeah, I exclusively make functional pots. It's a white stoneware clay, fine grog and not much of it. Not sure if you wanted more info than that, but it's the studio clay so I don't know much else. 

 
I don't know the firing schedule for either unfortunately. Chances are they are done pretty quickly because there are a lot of people at the studio. They pretty much always have a cone 6 and bisque going. I do know that they will unload the pots while still too hot to touch with bare hands if that makes a difference.
 
It's probably about 50 percent of the glazes that I get pitting with BUT sometimes a glaze I usually have no issue with will pit. And the pitting is not usually a cluster of pits, it's more random than that. For example I got 4 mugs all glazed the same way and they each have about 5 pits in random spots.

 

I feel like 70 percent of the pots I glaze have at least 1 pit. And I keep getting mixed reviews about whether pitting is terrible or okay to sell if it's barely noticeable. 
 
I know some other people also have pitting, but I don't get the chance to really check out others pots. 

 

Yeah I get the pinholes that I smooth over, but the problem is the pitting I get in the glaze firing.
 
Pictures: 
1st two are of a mug I just got out of the kiln (sorry these pictures aren't great)
 

 
2nd

 

Why is it bad to sand greenware?
 
Also, I wish I had more info but I'm still learning. I'm good enough that I feel comfortable selling my work, but the studio does all the firing and glaze making.

 

Thanks!

[oceancity]

Also as far as the firing goes...I have been reading and it seems that some people do a soak or hold at temp. and this can help pitting. I know the studio fires the kilns at night and they are electric. I don't know if that can be programmed in or not. But I'm assuming they don't do a hold unless it can be programmed into the kiln settings because they aren't there while it fires.

[Rae Reich]

It's not bad for the Pots to sand bisqueware or dry greenware, it's hazardous for your lungs. You can't help breathing those fine particles which can't be broken down or flushed from your body. Smoothing the clay while its damp or with a dampened sponge when dry is much safer for you and the potters around you. If your goal is perfectly smooth bottoms, wet sanding your finished pots is a good choice.

Good luck

Rae

[Min]

Re using a ^6 - 10 range clay for functional pots. 

 

That is too broad a range of maturing temps for functional pots. It’s either going to be immature at the low end and / or over fired at the top end. Simple test, make a cylinder with a flat base and put it in the next glaze firing with no glaze on it. Then take the fired pot and fill it with water and place it on a sheet of newsprint. Leave it alone for a few days. Check the newsprint for any wrinkling. I’m guessing it will be wrinkled which would mean your functional pots will leak over time. Also, the unglazed base will soak up water from washing, which will cause eventual crazing of the glazes and also cause problems when used in a microwave.

 

Re pinholes and pitting in fired glazes. 

 

Pinholes and pitting are usually caused by a couple things, firstly, something in the clay that is burning out and off gassing through the glaze. Secondly, something in the glaze itself that is off gassing. The glaze hasn't been given enough time to heal over, different materials off gas at different temps but can usually be smoothed over at the top temp. (Can also be because the glaze is too stiff but I don’t think its this since you are getting mixed results of okay and pinholes. You can also get pitting over heavily grogged clay, especially where it is trimmed but I don’t think thats the culprit either with the description you gave of your clay.)

 

I would ask the tech who fires the kiln for the firing schedules, the cone they bisque to plus if the kilns are well vented, downdraft vent? If they are leaving the kilns to fire unattended overnight then I’m guessing they have an electronic controller so it should be possible to add a soak at the end of a glaze fire to give time for the pinholes / pits to heal over. I’ve added a second soak to my firing schedule on the way down, it has helped reduce pinholes in one of my problem glazes.

[Tarantara]

Min, your response is interesting to me. I'm just now working on learning the intricacies of glaze firing, and have had issues with pitting in my first two loads.

How long to you soak (I am firing to ^6 with Amaco glazes, on ^6 white clay) on your first and second soaking? I do now have a new kiln with an electronic controller (was simply a kiln sitter, a pyrometer, an old kiln, and me having quality time together) to help me out. Right now I feel like I did while learning to make fudge - it's all alchemy...sometimes the magic happens and you get something luscious; sometimes you have something to be desired....

[GiselleNo5]

Min, your response is interesting to me. I'm just now working on learning the intricacies of glaze firing, and have had issues with pitting in my first two loads.

 

How long to you soak (I am firing to ^6 with Amaco glazes, on ^6 white clay) on your first and second soaking? I do now have a new kiln with an electronic controller (was simply a kiln sitter, a pyrometer, an old kiln, and me having quality time together) to help me out. Right now I feel like I did while learning to make fudge - it's all alchemy...sometimes the magic happens and you get something luscious; sometimes you have something to be desired....

 

Several years in and I'm just now starting to have more than half of the glaze loads turn out the way I want .... the making portion is so much easier in my opinion. And I'm using commercial glazes ..... theoretically should be easier? LOL 

 

"EASIER"

[Tarantara]

 

Min, your response is interesting to me. I'm just now working on learning the intricacies of glaze firing, and have had issues with pitting in my first two loads.

 

How long to you soak (I am firing to ^6 with Amaco glazes, on ^6 white clay) on your first and second soaking? I do now have a new kiln with an electronic controller (was simply a kiln sitter, a pyrometer, an old kiln, and me having quality time together) to help me out. Right now I feel like I did while learning to make fudge - it's all alchemy...sometimes the magic happens and you get something luscious; sometimes you have something to be desired....

 

Several years in and I'm just now starting to have more than half of the glaze loads turn out the way I want .... the making portion is so much easier in my opinion. And I'm using commercial glazes ..... theoretically should be easier? LOL 

 

"EASIER"

 

LOL

 

Alchemy.

[Min]

Min, your response is interesting to me. I'm just now working on learning the intricacies of glaze firing, and have had issues with pitting in my first two loads.

 

How long to you soak (I am firing to ^6 with Amaco glazes, on ^6 white clay) on your first and second soaking? I do now have a new kiln with an electronic controller (was simply a kiln sitter, a pyrometer, an old kiln, and me having quality time together) to help me out. Right now I feel like I did while learning to make fudge - it's all alchemy...sometimes the magic happens and you get something luscious; sometimes you have something to be desired....

 

Tarantara, I'm happy to give you my firing schedule with the double soak. It works with my kiln(s) and glazes but you would have to try it and probably tweak it to work with your kiln / glazes. If you do give my schedule a go please use witness cones to adjust soak times to bring your cone down to how it works for you. (temps in Fahrenheit)

 

250 hr to 250 no soak  (I sometimes fire pots when glazes still damp so this slow rise dries them out)

350 hr to 1900 no soak (sometimes I increase this to 400 hr)

108 hr to 2180 soak for 15 (cone 6 tip just barely touching shelf)

9999 hr to 2100 soak for 15

200 hr to 1400 no soak (i use this step so my smaller kiln cools similarly to my big kiln so my semi matte glazes look the same)

 

(psst, a really good fudge recipe is the Crazy Maime one, it never fails or turns into chocolate soup)

[Tarantara]

Thank you, Min. I shall give this a whirl soon and see what comes out of the kiln.

And thanks for the fudge recipe suggestion. Fudge is one of those things that is transcendent when all goes perfectly, but you can always put Failed Fudge on ice cream. Failed pots are another story. Those you just chalk up to the learning curve.

[glazenerd]

 

It's a white stoneware clay, fine grog and not much of it. Not sure if you wanted more info than that, but it's the studio clay so I don't know much else.

Looking at the size of the pinhole on the first mug (laid over), I would be looking at the clay. Min noted clay issues causing pinholes, and she was spot on.  I not only see the pinhole, but a raised area around it (glaze usually does not cause that.) I also see some raw materials up near the rim; which should not be there.

 

Nerd

 

Now with my coffee in hand, I will walk out to the studio and listen to the bird symphony going on outside.

[Pugaboo]

I would try a simple holds to start.

 

Add a hold at the end of the bisque schedule, if the kiln is really packed you might not be getting an even cone 04 in all areas of the kiln which could give you random issues with glazes because all the impurities have not burned out of the clay. Once you do this, test a few pieces, do they still throw random glaze imperfections?

If so...

 

Try adding a hold to your glaze schedule.

 

I had one glaze that gave me random pinholes no matter what I did UNTIL I added a soak at the end of my glaze firing schedule. It took some playing around with ^5, 6 and 7 on ALL the shelves for every firing until I got the issue solved and then I marked what a perfect bend looked like for that schedule. For me I had to do a cone offset with a hold to get perfection. Now every time I fire I do a little drawing of the cones bend on my kiln log page. If I get a cone that hasn't bent "perfectly" I make note of it, if it happens twice I know there is an issue I need to deal with.

 

Telling you to add a 5, 10 or whatever minute hold to your schedule might not necessarily work for your work, glazes, kiln, firing schedule. It's a good starting point but you need to realize every kiln fires a bit differently depending on a lot of factors. You will need to decide I am going to try a ___ minute hold, then check your cones and pots and see if you have achieved what you need to. When doing this it's important to use a cone under (5), target cone (6), and a cone over (7) for a cone 6 firing, so you can see where you are from your target temperature and adjust accordingly. You might even want to record where each piece was located in the kiln, that way if it's the same spot in the kiln giving you pots with holes then you know you have a cold spot and that particular glaze needs a hotter spot to smooth over. I also mark on the side of each used cone the date and location it was used in the kiln so I can compare if I have future issues.

 

Is it possible to be there when the kiln is loaded? You can learn lots by just knowing where your pieces are placed in the kiln. Learning how to load and program a kiln is very valuable information. Before I got my own kiln I volunteered whenever I could to help load the kiln, I learned a lot. I ALSO learned I wanted my own kiln so I could control my results better, nothing like putting in all that work just to lose it because your piece was set someplace it didn't like in the kiln, or next to another piece that reacted to it badly.

 

You do know you can grinding the bad spot (wear a mask!) reglaze that spot and re-fire yes? It sometimes works sometimes doesn't but I did this while tweaking my holds and learned a lot about doing that too in the process. You can't just put more glaze over the bad spot as it tends to give you a reglazed bump, I found grinding off the offending spot then heating up the pot and dabbing new glaze on it worked well.

 

As for SANDING bone dry pottery PLEASE DONT DO IT! Google "silicosis", it's not something you want to develop. By sanding your pieces you are putting lots of fine particles in the air and they go straight into your lungs... And stay there. The particles can remain airborn for up to a day and they settle on all the surfaces around you only to be reintroduced to the air when someone else uses those areas. Sanding bone dry pottery is not only dangerous to you it is also dangerous to those around you.

 

You said your clay has very little grog so you should be able to use a wet (damp) sponge to easily smooth out any rough spots on your pots without exposing any grog. You will be amazed at how well it works. IF you absolutely must sand or abrade the dry clay surface do it outside away from the building while wearing a protective mask and ask others to do so as well or to leave the area.

 

T