Bisqued slip is running during glaze firing

[MKG001]

I brushed a light-colored stoneware slip onto a leather-hard Laguna B3 brown piece and bisqued at Cone 04. Then I brushed a clear glaze (Amaco Clear Mixing Celadon) onto the piece and glaze-fired at Cone 5.  To my surprise, the slip seems to have come loose and redistributed over the surface, rather than retaining its original pattern.   See before and after photos.

Is this typical? Or maybe something to do with the glaze I used?

Sorry if this is a newbie question, I am a newbie :-)

Thanks!

[neilestrick]

The slip has not moved. The color of the brown clay is coming through the slip where it is thin. You need more coats of slip.

[MKG001]

Why does the brown clay show so much more after glazing?

[kswan]

As pieces are fired, the glaze and top layer of the clay are literally melting into one another. In this case, it's your slip. Where it's thicker, it still has enough coverage of the clay underneath that the top part melting into the glaze doesn't show. You need that zone to melt together to make a stronger glaze fit. 

[MKG001]

Thanks, this is super useful for me to understand.

[neilestrick]

If your white slip is a porcelain body then it might be somewhat translucent, so a thicker layer is needed to make it more opaque. Plus the iron in the dark clay is pretty powerful stuff and will tend to darken just about anything put on top of it that's not very opaque.

[MKG001]

Thank you. The white slip is some random stoneware mix from a local supplier :-)

[Rae Reich]

If there is another potter who has not made this mistake at least once, I’ll eat a bug! Testing and practice will get the results you want.

Also, porcelain is often classed as a stoneware clay, so there could be porcelain in a white stoneware mix slip. It’s not a slip problem, it’s an application problem.

You can learn to love this piece, even if didn’t come out as you planned, it has a misty quality. 

[MKG001]

Thank you so much for your kind reply! I really appreciate it.

[Kelly in AK]

There are many potters who skillfully use this effect to their advantage. A couple of Matt Kelleher’s lovely pots, white slip on dark clay:

[MKG001]

Wow, those are lovely. Thanks for sharing these examples! 

I think I'll say my egg came out exactly the way I planned ;-)

[oldlady]

mkg, glad you like the results.  may i suggest something that may not have occurred to you?  i think the lines you drew were done with a sharp instrument like a needle tool.  

the raggedness of the lines indicate a sharpness to the pattern that could cut skin if someone rubbed it.  the shape invites rubbing so someone may be cut on the edges.  to avoid that, use a stylus with a ball end that is just the right size or look at Diamond Tools for their special cutting tools for carving and scraffito.

[MKG001]

Hi, you are exactly right. I will definitely check out those tools. Thanks so much for your advice!

[neilestrick]

11 hours ago, oldlady said:

to avoid that, use a stylus with a ball end that is just the right size or look at Diamond Tools for their special cutting tools for carving and scraffito.

A ball point pen works well, too.

[Chilly]

On 12/17/2022 at 5:17 AM, oldlady said:

raggedness of the lines indicate a sharpness to the pattern that could cut skin if someone rubbed it

As @oldlady rightly says, (the sharp scratchy bits are called boogers in my studio).  I tell students that the finish will never be better after firing, so make sure it's as good and smooth as it needs to be before firing.