What is this defect? And how can I avoid it?

[Crooked Lawyer Potter]

What do you call this glaze defect?  Bisqued at 06, electric fired at 6.  Long dip of Fat Cream with an ash glaze rim.  BMix.  It seems to happen randomly. 

[Callie Beller Diesel]

That glaze defect is crawling, which can happen for a few reasons. In your case, it looks like the application of the fat cream went on a bit thick. 

[Crooked Lawyer Potter]

So ... lower the viscosity? or lower the SG?  Or a quicker dip?

 

[Pres]

@Crooked Lawyer PotterThin the glaze a bit and make a quicker dip.

best,

Pres

[Crooked Lawyer Potter]

I may have asked this before, but this business of "viscosity v specific gravity v deflocculation" keeps me in a state of confusion. 

For instance, if I want to "thin" the glaze I could: (1) add water - thereby thinning it and lowering its SG, or (2) deflocculate itthereby thinning it but keeping its SG the same, right?  Is there a reason to do one over the other?

And, is "thinning" it the same as making it more "viscous"?

 

[Mark C.]

I always just add wateer to thin it.. Once you fine the right specific gravity (I write that # on side of bucket)

[LinR]

And make sure your pot has no dust or oil (from your fingers) on it before you glaze.  Lin

[Bill Kielb]

15 hours ago, Crooked Lawyer Potter said:

And, is "thinning" it the same as making it more "viscous"?

My suggestion thin it with water until you find a nice thickness for your application habits. It looks applied too thick so how can you apply it thinner?
Maybe think of viscosity as the internal friction of the fluid. The more viscous, the less likely to flow freely or (it’s thicker) So adding water or thinning definitely will reduce the viscosity or make it flow more freely. There are other reasons for crawling but your pictures indicate it was applied too thick.

Before jumping into other remedies, maybe try the simplest by adding water till the glaze works for your habits. +1 for finding this ideal personal sg by testing  and measuring the specific gravity and …….. marking it on the bucket.

[Callie Beller Diesel]

Before you adjust any large batch of glaze, it’s a good idea to take a sample out and work out the problem on that. I agree with Bill that often glazes do just need a bit more water, especially if you’re working at cone 10. 

Sometimes though, thinning the glaze in the bucket makes the fired result less desirable. It’s not necessarily wrong, but maybe you don’t like the look of it. In that case you want to adjust how it flows, or the viscosity, with a deflocculant to make it more fluid (darvan) or a flocculant to make it thicker (epsom salts). 

As to why you’d want to adjust the viscosity of a glaze with water vs flow adjusters, Sue McLeod posted this the other day, and it’s a concise description with lots of images describing just that. (It should open in your browser, even if you don’t have the instagram app.) https://www.instagram.com/p/Cn2dMQHOTNP/?igshid=YmMyMTA2M2Y=

[Wayne2]

One can also add a bit of gum solution to fix crawling in a dipping glaze. You don't want to add too much so better to try on a smaller sample.

This may not work if you have lowered the specific gravity though See https://digitalfire.com/material/228

Note that if you buy any prepared gum solution it may not be the same as the gum solution discussed here. A bit of some bactericide such as Detol to prevent the bacteria from eating the CMC is needed if you store the gum solution (or the modified glaze).

[Jeff Longtin]

Another aspect you might look at is temperature. Is this happening in specific areas in the kiln? 

You might drop a few cone packs into your kiln to see if there are hot spots. If the clay body starts to vitrify, before the glaze has melted, the glaze will "run off" (crawl).

Most likey an application issue but over firing can sometimes be the culprit as well.

Also, have you allowed the pots to dry thoroughly before firing? This happens to me a lot, with my slip cast mugs, if I fire before the glaze has dried. 

[Crooked Lawyer Potter]

11 hours ago, Jeff Longtin said:

Also, have you allowed the pots to dry thoroughly before firing? This happens to me a lot, with my slip cast mugs, if I fire before the glaze has dried. 

Curious about this idea.  I DO often start up the kilnfiring without letting the glaze get good and dry, but I always start the fire program  with a 2 to 5 hour pre-heat that I assumed would take care of the issue.  What do you think?

[Jeff Longtin]

3 hours ago, Crooked Lawyer Potter said:

Curious about this idea.  I DO often start up the kiln firing without letting the glaze get good and dry, but I always start the fire program  with a 2 to 5 hour pre-heat that I assumed would take care of the issue.  What do you think?

I would say this is a good place to start. Once I let my mugs sit overnight the problem went away.

I think the problem with expecting them to dry, during the firing, is that they get too hot, too soon. Allowing them to dry, outside of the kiln, allows them to dry completely before heat enters the picture.

[Crooked Lawyer Potter]

Will take this to heart.  I love pottery but it does not match well with my congenital lack of patience. 

[Mark C.]

You can only work as fast as the clay will let you grasshopper

[Callie Beller Diesel]

You’re not alone. It’ll teach you patience whether you want it to or not!