How to control blurring of oxide under glaze or over glaze decoration?

[Franny]

Hello

I've  recently started experimenting with using oxides for under glaze or over glaze decoration.  I've searched the site and found many helpful tips, but none have answered my specific queries (below).

I'm aiming for subtle line effects and am testing with different glazes.  Currently I'm simply mixing tiny quantities of oxides, similarly tiny quantities of china clay, and water (also testing with using matcha tea instead of water, as I read somewhere that this can make application smoother!!)  - but I'm not adding any kind of frit.   I'm testing on greenware, and on bisque - under, or over glaze (sometimes referred to as in-glaze, on-glaze, or majolica)

From the test results I've had so far I'm drawn to the effects of under glaze oxide application, using glazes that are semi-transparent, or translucent, rather than fully transparent, and will continue to experiment, including with layering glazes over the oxide decoration to achieve the 'shrouded' effects I'm looking for.  Intuitively, and from my test results to date, using oxides under glaze is getting me closer to this 'shrouded' effect, than using them over or in-glaze - but I'd be really happy to hear your opinions and suggestions about each approach.  

So, I will continue testing but meanwhile I'd really appreciate some wisdom on a couple of issues:

The first is how to  control the movement/blurring of the oxide decoration.  I've come across suggestions of adding gum arabic or glycerine to the oxide mix to 'fix' the decoration in place, but suspect this will simply burn out in the glaze firing and the oxide decoration will still move as the glaze melts.  I suspect that controlling movement and blurring of the oxide decoration may be more a matter of finding the sweet spot of glaze melt-fluidity, and this may involve increasing the alumina to silica ratio in the glazes used over the oxide decoration.  Am I on the right track?  I would really appreciate your thoughts and tips.  

The second issue concerns how to ensure that the oxide decoration is fully 'encapsulated' in the glaze to ensure, particularly with toxic oxides such as cobalt, that any potential leaching is minimised.

Many thanks, and really looking forward to your thoughts and suggestions! 

 

 

  

[Min]

18 hours ago, Franny said:

The first is how to  control the movement/blurring of the oxide decoration.  I've come across suggestions of adding gum arabic or glycerine to the oxide mix to 'fix' the decoration in place, but suspect this will simply burn out in the glaze firing and the oxide decoration will still move as the glaze melts.  I suspect that controlling movement and blurring of the oxide decoration may be more a matter of finding the sweet spot of glaze melt-fluidity, and this may involve increasing the alumina to silica ratio in the glazes used over the oxide decoration.  Am I on the right track?

I would agree. Fluid glaze vs stiff glaze will make a difference as to the colouring oxide movement. If you think of a majolica glaze for example, they are very stiff glazes with very little movement. Another factor will be which oxides you are using, copper for example will be very difficult to keep with a crisp edge, stiff glaze or not. If you find just using china clay + colouring oxides + water/matcha doesn't give you the effect you are looking for you could try underglazes, either in prepared form or powdered that you add your own medium to.

18 hours ago, Franny said:

The second issue concerns how to ensure that the oxide decoration is fully 'encapsulated' in the glaze to ensure, particularly with toxic oxides such as cobalt, that any potential leaching is minimised.

I think common sense plays into this aspect insofar as brushwork over or under a glaze and the quantity of the colouring oxide present. Only way to know for sure how much leaching is happening would be to have a sample lab tested. Before doing that I would do a basic acid and base leach testing yourself, this will rule out ones that are definitely leaching,  they could still be leaching a small amount that isn't visible to the naked eye. (Home tests rule out glazes, don't necessarily rule in glazes)

Welcome to the forum!

Edited October 26, 2022 by Min

[Rae Reich]

I like the sound of the effects you’re going for!
I’ve had some success decorating under and over glazes with oxides and stains by mixing the colorants with (possibly thinned) glaze - either the same glaze or, if that’s too fluid, with a stiffer glaze. This helps the color meld better with the glaze, eliminating those rough spots that occur when oxides are applied too densely. Do some testing, applying them over and under your base glazes to see what variations you might want. I’ve used both clear and opaque base glazes added to the colorants, as well as whites with and without talc (which makes lavenders/lilacs of cobalt. 
My friend/mentor potter, Gary Zeiner, always added a pinch of tin to his signature blend of RiOx/rutile decorating oxides, so I do, too. 
I’m an Old School ^10 potter, so my base glazes for colorants are variants of 1-2-3-4 Clear, Watts White and MG2 matte. 

Edited October 27, 2022 by Rae Reich

[Franny]

Thank you, Min and Rae 

Since posting I've run more tests with three of my 'studio' glazes, and have been really pleasantly surprised!  

Previously, my testing had been on test tiles - I make small cones that I cut into three so each portion has a base it can stand on in the kiln.  This give me the option to use both front and back of the test tile to get as much info as possible!  I used most of the oxides in my stores, mixed as described with china clay and matcha, applying these to raw, leather hard clay on the front of each test - a single line, and some brushwork; then on bisque on the back, again a single line plus brushwork; and applied the same relatively fluid glaze to all these tests.  The results were quite mixed and most of the oxide decoration looked as if it was blurring.

However, I've now tested again on the exteriors of three bowls, using cobalt, copper, RIO, and rutile on bisque, aiming for light and delicate brushwork.  I thought that at least two of the glazes used would be excessively fluid, but the results are more encouraging than expected - always nice when this happens!  Not yet achieving the 'shrouded' look I'm aiming for, but now looking forward to exploring glaze formulations to achieve this.

When I mentioned using matcha tea as a medium for mixing oxides I couldn't remember where I'd seen that reference - or rather it was in an Instagram post that I now can't track down.  But I came across this on Richard St John Heeley's website http://www.richardheeley.com/index.html/blue_%26_white.html :

‘Gosu’ is the Japanese name for natural cobalt. The ground cobalt powder was traditionally applied with a thick green tea solution that had been boiled down to a syrup. This mixture would be combined with the ‘Gosu’, enabling it to be applied freely and smoothly during painting. It was allowed to dry and then glazed. The tannin in the tea kept the decoration from spreading when the glaze was applied.

Interesting!  Don't you just love chemistry?!! I sense a google rabbit hole search of 'tannin' may be on the cards!!! Happy days

 

[oldlady]

franny, do you have any photos of what you are trying to achieve?   sorry, i am one of the chemical idiots and do not plan to embrace it.   if there is a potter out there whose work you admire, a photo or website would be helpful.

yes, the chemists among the membership know a huge amount about combining things to get a particular result.  someone will know what you need to do.

[PeterH]

45 minutes ago, Franny said:

When I mentioned using matcha tea as a medium for mixing oxides I couldn't remember where I'd seen that reference - or rather it was in an Instagram post that I now can't track down.  But I came across this on Richard St John Heeley's website http://www.richardheeley.com/index.html/blue_%26_white.html :

‘Gosu’ is the Japanese name for natural cobalt. The ground cobalt powder was traditionally applied with a thick green tea solution that had been boiled down to a syrup. This mixture would be combined with the ‘Gosu’, enabling it to be applied freely and smoothly during painting. It was allowed to dry and then glazed. The tannin in the tea kept the decoration from spreading when the glaze was applied.

Interesting!  Don't you just love chemistry?!! I sense a google rabbit hole search of 'tannin' may be on the cards!!! Happy days

I have a faint memory of stale 7-up being used as a medium by some Japanese potters.

OILS AND MEDIUMS  http://www.porcelainpainters.com/mediums.htm
Sugar
There are numerous recipes for sugar solutions to be used for pen work and for painting. The most common and easy medium is regular 7-Up. Everyone who paints or draws with sugar solutions seem to have their favorite mixture. The most common recipe is made by boiling 2 parts water with 1 part sugar, and using the syrup as a painting medium or thinned for pen work. It dries quickly and requires practice to gain consistency in brush work.

[Min]

17 minutes ago, PeterH said:

I have a faint memory of stale 7-up being used as a medium by some Japanese potters.

OILS AND MEDIUMS  http://www.porcelainpainters.com/mediums.htm
Sugar
There are numerous recipes for sugar solutions to be used for pen work and for painting. The most common and easy medium is regular 7-Up. Everyone who paints or draws with sugar solutions seem to have their favorite mixture. The most common recipe is made by boiling 2 parts water with 1 part sugar, and using the syrup as a painting medium or thinned for pen work. It dries quickly and requires practice to gain consistency in brush work.

Is the op painting on fired glaze or under and over unfired glaze? I read their post as unfired.

[oldlady]

min, the test tiles are first done as greenware on one side then bisqued and tested on the reverse.   pretty clever1

Edited October 30, 2022 by oldlady