Engobe Or Underglaze?

[Maria Correia]

What is the liquid used in this slide. Engobe or Underglaze?  

https://www.youtube.com/watch?v=_PyKDYCNzF4

[Norm Stuart]

The potter in the video has probably mixed the clay he is using with a Mason Stain colorant or metal oxide or carbonate colorant. Since the clay bowl is still very wet, this slip will adhere fairly well.

But this colored slip layer can sometimes become detached or cracked as the bowl dries, or when it is bisque fired. This is why slip-trailing recipes always add a flux to help melt the slip to the bowl in the kiln. There's not much difference between a slip-trailing recipe and an englobe recipe.

I wasn't surprised when I read a page on the Frog Pond Pottery site (John Hesselberth co-author of "Mastering Cone 6 Glazes") which tested commercial under-glazes and found almost all of them blister-off the ceramic when fired. This is why englobes and slip-trailing recipes often contain flux material, something that melts in the kiln to glue the color to the clay.

Underglaze is a colorant (Mason Stain, metal oxide, or metal carbonate) mixed with clay (kaolin and silica) and water. Clay doesn't melt nor do colorants. Kaolin mixed with colorant will also has a different drying and thermal expansion characteristic than each clay that you use.

After reading Hesselberth's tests, we began making our own "underglazes" by mixing two parts of Laguna Gerstley Borate with one part Mason Stain colorant and six parts water.

This is not a glaze, but the boron in the gerstley borate melts just enough to securely adhere the color to the greenware or bisque when it's bisque fired. Even if it is slightly detached when dry, it will lay down and adhere in the kiln. People can then apply low-fire or high fire glazes over these colors, often a clear, and the colors don't run or mix into the glaze.

We also mix these color concentrates with a low-fire or high-fire clear glaze to make a colored glaze.

The flux materials added to the trailing-slip in this Ceramics Daily article are:

Talc
Calcium Borate (or Frit 3110)
Nepheline Syenite
Borax

http://ceramicartsdaily.org/ceramic-supplies/ceramic-raw-materials/...

The "underglaze" ingredients are:

Kaolin
Calcined Kaolin
Silica
Zircopax

If you use only the underglaze ingredients without adding the flux ingredients, you will probably encounter problems with the slip blistering-off the bowl.

Just start using your englobe and you'll quickly see if you need to make any changes.