Peeling slip after glaze firing

[Susan McHenry]

Does anyone have any input on why I’m getting intermittent peeling slip on my cone 1 red earthenware pots? It happens on a handful of pots from each firing. Typically, it’s on the edges of a pot (mostly near the rim). I’ve tried reformulating my slip recipe and am now using deflocculated slip (Martina Lantin’s recipe). My colors are from the base recipe with the addition of mason stains. I use Laguna red earthenware clay and fire to cone 1.  I hoped the deflocculated slip would solve the problem since the water content is so low, but it hasn’t. I’m ready to give up! It’s so frustrating. I’d greatly appreciate advice on this. See the attached photo. 

[liambesaw]

Have you tried not glazing one an seeing if it happens?  Could be the CoE is too different between the glaze and slip and body.  Just a guess since that is a low fire slip recipe.

Here is a thread where low fire slips are discussed if you want to try fishsauce slip, the recipe is a few posts down.  

 

[Babs]

I think that the rims are drying out a bit too much before applying the slip.

It may still look fine after the bisque but separates more during the glaze firing.

I run a brush dipped in water over entire pot prior to appying slip. This helps the laying on of slip and helps moisten pot all over.

Hope this helps...can cover rims with ceram wrap  to avoid rims becoming drier than rest of pot

[Susan McHenry]

Thank you. It may indeed be a CoE with my glaze. Thanks for the suggestions. I'm going to try to reformulate the glaze and see what happens. I did a bit of reading on glaze fit. I'm going to reduce the amount of silica and increase the amount of feldspar and run a few test. 

[Min]

I would try checking one variable at a time. Try the clay with the slip but no glaze like Liam suggested but also try a test piece with no slip and thick glaze. If you have any test cylinders try glazing the inside and rim thickly then after firing put the test pieces in the coldest part of your freezer overnight. Next day put them in the sink and pour boiling water into them. Check for shivering and tap along the edges with something like a knife handle and see if any (more) pieces pop off.

Does that glaze craze? If it does then the glaze is not the problem and you can skip the above test for the glaze but I would still do it with just slip on the clay. Is there any sign of the slip shelling when the pots come out of the bisque?

[glazenerd]

Hi Susan:

from reading your original post: (whitish) slip over a red bodied clay. In your picture; there is a piece of visible red clay body, covered with slip. ( no glaze). It is one thing for glaze to shiver, another for slip to peel: but to lose chunks of the red clay body? 

CoE is at play in part: but it would take a serious amount of contraction to blow off chunks of the body. Red bodied clay as you show would have nearly 50% Newman red,  Imco burgundy, or red art to achieve that color in a recipe. Most likely Newman red given your clay supplier. That said: 50% of red clay would supply enough iron (disulfide) to cause coring; or at minimum brittleness if fired to quickly. The rims would be more susceptible due to thickness and exposure to heat. 

Could you post your firing schedule please? If you bisq first, please include that firing schedule as well. Something is off here: including slip application " appears" to be heavy; but that would translate to COE issues. What is the cone rating for the red clay? 

Tom

[Susan McHenry]

Hi Tom,

I use Laguna red earthenware clay, EM-207. They don't list a cone rating. I've contacted them and they said it's up to the user to determine the cone temp by testing (strange). I used to fire Standard's Red Earthenware to cone 1 (the firing range for that body is 06-2). I prefer the  Laguna to Standard's which caused scumming issues. Laguna adds a small amount of barium to prevent scumming. That said, I used to have the same issues with peeling using the Standard clay. I've tried several slip recipes, thinking that may have been the problem, but that hasn't resolved it. Maybe it's my glaze recipe? I can't recall the source, but here is the recipe:

Neph Sye 46

Gerstley Borate 28

Silica 20

EPK 6

Bentonite 1

 

The white slip was a bit heavier than is typical for me on the pieces in this firing. That's because I started deflocculating my slip, thinking maybe I was getting peeling from too much water in the slip. So, I'm now getting used to how to tweak the slip to the right consistency. That may be one issue, as you say. 

As for my firings, I bisque to cone 04 on a slow bisque cycle. It takes about 13 hours. I run a slow glaze program to cone 1 that takes about 7.25 hours. 

Thanks so much for your help!

[glazenerd]

Hi Susan:

After reading up on EM207; think I have figured this out. This clay has up to 65% talc; and a cone fire of 06 (1830 +/-). You are firing to 2080 (cone 1). Talc is used in low fire bodies for several reasons: primarily to control thermal expansion. Typically talc is added in the range of 15-25%; to increase the COE  the clay body into the range of typical glazes. However, when talc additions are made in the 50-65% range it has the opposite effect of lowering the COE. I will run their analysis later to check it; but expect to see ovenware range COE of 4.50 or so. Just looking at your glaze recipe without running it on a calculator- it is probably in the 8.50-9.00 range. 46% Nep Sy and 28% g.b. Is a boat load of flux.  Look at it this way: 74% fluxes and 26% minerals ( silica and EPK). 

For the record: given the analysis given on the MDS sheet: the red is being supplied by synthetic red iron oxide. 

So yes you have a COE issue: extreme enough to blow pieces off. This is compounded by over firing a cone 04 clay to cone 1. Further compounded by the very high talc ratio: which includes 37% magnesium content: an auxiliary flux at higher temps. You will not be able to fix this without changing clays, or changing slip and glaze to fit this clay at 04.  Sorry for the bad news.

Tom

calculated COE:   Red clay: 4.89.  Glaze: 7.85  ( clay would probably test lower)

[Susan McHenry]

I really appreciate your looking into this Tom. I wish I could fully understand your calculations regarding COE. Sadly, I'm not well versed in clay and glaze chemistry. I get the gist of what you're saying though. I will test fire at a lower temperature and will test some other low fire glazes. From what I'm understanding, the glaze I've been using is not the best fit. I know there are several Val Cushing low fire glaze recipes out there that may work better for me. Thanks again. 

[glazenerd]

Susan: both your slip and glaze Needs to be adjusted to fit. I started an " understanding COE" thread to help you and others. COE questions come up on a weekly basis. 

[Babs]

It's only happening "To a handful of pots per firing" right?

Have they the same form?

Are the same shelf of kiln?

Are they the same dryness when applying slip?

Only a handful...check that out....

I wouldn't be fiddling with glaze etc till that is sorted.

[glazenerd]

Babs:

next time I talk to John Baymore: going to harass him for starting the " test, test, test." Given this is an 06 clay being fired to c1: that has 65% talc in the body limits my focus. I will venture out on a limb and state: the only difference between the rims which have broken off and those that have not- is time 

[Babs]

Yeh but..only a handful of potssss........

Her glaze is a cone 3 which she is firing to cone 1.....

Her clay is...

Lots of enigmas here.

And because as JB said more than once.   "It depends ..."

Has OP kept any good pots to see how they age?

I would start where it is easy.   Where how and when slip of constant mixture is applied.....and is successful. Replicate that...

Still hoping John gets bored   Ha ha ha or retires from a few of his activities and is driven to sorting us out again..