pinholing Terracotta & glaze compatability

[maidmarion]

Hello, 

I have been making terracotta tiles & glazing them in the Majolica style. The tiles were bisque fired to 950 and the 3 coats of white glaze applied to tiles with decoration painted on the surface. The white glaze was a white glossy glaze not a Tin Glaze. 

Pin holing was apparent on surface of glaze before firing and remained after firing to 1080.

Anybody got any advice please ? 

[Min]

Hi maidmarion and welcome 

Majolica glazes tend to be stiffer glazes so they don’t move much in the firing, this makes pinholes harder to heal over. The pinholes in the raw glaze should be rubbed smooth before firing. Do you rinse off or sponge down the pots and let them dry most of the way before glazing? This can help reduce the number of pinholes in the raw glaze from the air leaving the porous bisque. A soak at the top temperature plus a soak at about 40C below the top temp for 15 - 30 minutes as the kiln is cooling can help them heal over without over firing the clay/glaze. If you are using a groggy or sandy clay I would burnish it after trimming as the pits left in the clay can promote pinholes also.

Getting pinholes to heal in flat surfaces like your tiles is more difficult than on vertical surfaces. Is 1080 the top temperature you can fire this clay? What temperature is the glaze rated to?

[maidmarion]

Hi Min,

Thank you for your swift & knowledgable reply - I an fairly new to clay so am having to learn at a fast rate !

Your answer made sense, No, I did not wipe tiles before glazing so that could be worth a try.

I also used a Botz white shiny glaze as a base as I'm a little nervous of mixing the Tin Glaze powder and you don't seem to be able to buy it ready mixed.

Burnishing at the leatherhard stage makes sense too, I used a 20 percent grogged terracotta.

I used Amaco underglazes & stains on top of base coat  - alas I only applied 1 coat so they came out a tad pale.

[Marcia Selsor]

Agreed- rub /sand all defects from the bisqued clay. then, wash the tile with diluted vinegar  before you glaze. allow the tiles to dry before glazing. Learned the vinegar wash from Rosalie Wynkoop, my favorite Majolica artist.

 

Marcia

[maidmarion]

Thank you Marcia - I must say you are a very helpful lot !

[andros]

On 23/12/2017 at 7:02 PM, maidmarion said:

Hi Min,

Thank you for your swift & knowledgable reply - I an fairly new to clay so am having to learn at a fast rate !

Your answer made sense, No, I did not wipe tiles before glazing so that could be worth a try.

I also used a Botz white shiny glaze as a base as I'm a little nervous of mixing the Tin Glaze powder and you don't seem to be able to buy it ready mixed.

Burnishing at the leatherhard stage makes sense too, I used a 20 percent grogged terracotta.

I used Amaco underglazes & stains on top of base coat  - alas I only applied 1 coat so they came out a tad pale.

For majolica I used a normal white glaze (not tin glaze) and everithing was ok... as I told in another post I aways applied one coat of underglaze pretty diluted, according to italian tradition. In this way colors don't appear "solid" and uniform like those applied with multiple layers but anyway they appear bright... Anyway I understand  that normal underglazes and stains used over a white glaze (tin or not) don't have usually enough flux, so it's necessary apply at least a thin layer of clear glaze as a finishing coat. Of course this layer should be applied by dipping or sprying because brushing would rub out the colors... Maybe this can be the reason for your pale colors? For majolica exists colors made expecially for that (called "overglazes") with more fluxes, but also many majolica artists in Italy uses underglazes with that precaution, so they are not forced to have may different kind of colors...

[Marcia Selsor]

Another thing my majolica artists friends do is fire hotter for the bisque than the glaze. They usually fire to ^03 or 02 for a TC bisque and glaze fire to ^04. 

In Centigrade depending on the heat rate climb and type of cone used that temperature varies for ^03  goes from1071-1101 C  and the majolica glaze for 04 is about 1040-1069 C

The higher bisque helps eliminate the pin holing for the glaze. ...according to them. This is advice from people like Rom Meyers and Linda Arbuckle. Ron uses a clear over underglazes, Linda uses over glazes and majolica. Both use terra cotta earthenware. Here is a link to Linda Arbuckle's website with its of handouts. Excellent information. http://lindaarbuckle.com/arbuckle_handouts.html

 

Marcia

[tinbucket]

My first guess is what Marcia said. Bisque to a higher temperature than you glaze fire to. Red clay has lots of impurities/organic matter which leads to a high loss on ignition and off-gassing (matter burns out of the clay and produces gas, the gas is trapped by the glaze).

[maidmarion]

Hello All,

Thanks for all your very useful and wise comments, I have made copious notes and look forward to trying out your suggestions. Happy New Year to you All.