Transparent glaze is not covering the cobalt oxide ... ?!?

[Guillaume]

Hello everybody. My name is Guillaume, I'm learning ceramic with the ressources I find here and there ... that's why I probably don't have the right vocabulary, but I hope we will understand each other °_____°

So, nowadays I'm trying to work with cobalt oxide. I'm not trying to find the right color yet, just being able to draw a clean line and put some glaze on it. The problem I got is that the glaze is not covering the oxide. It runs away from it.

What I used is 5% cobalt oxide mixed with ... water based acrylic paint. I did that because I wanted to fill an empty marker and draw (I don't like to work with brushes). I thought that it might be a good medium + the paint will burn away. 

Any idea on how to fix this ?
Thanks ! G.

Photo 1 : inside with glaze / Photo 2 : outside no glaze
 

[Babs]

So you are painting then glazing.

I am wondering if the acrylic paint is repelling the glaze coat.

It is possible to paint with just the oxide and water or mixed wiith a little of the glaze, or frit.can bedone prebisque firing. Thus stops any damage to the oxide pattern.

The oxide mix needs to be kept afloat so frequent , Giood luck

 

[blackthorn]

The acrylic is definitely repelling the glaze.  I use acrylic or gesso frequently as a resist on both green ware and bisque.

Everything Babs says is perfect and will solve your problem.

And welcome to the forum.  Lots and lots of useful information here and many experience crafts people as well.

[PeterH]

5 hours ago, Guillaume said:

What I used is 5% cobalt oxide mixed with ... water based acrylic paint. I did that because I wanted to fill an empty marker and draw (I don't like to work with brushes). I thought that it might be a good medium + the paint will burn away.

I think this still leaves the problems of applying home-made oxide washes with a marker-pen style technology. What would be a suitable medium? cmc??

[Min]

6 hours ago, Guillaume said:

What I used is 5% cobalt oxide mixed with ... water based acrylic paint. I did that because I wanted to fill an empty marker and draw (I don't like to work with brushes). I thought that it might be a good medium + the paint will burn away. 

Just to clarify something, did you do your cobalt mixed with acrylic paint on greenware then bisque fire it or did you glaze on top of the cobalt / acrylic paint without bisque firing it first to burn off the acrylic?

If it was bisque fired first then I think the crawling is because the cobalt is repelling the glaze. Cobalt can act as a flux in glazes but used in high amounts under a glaze it can cause the glaze to crawl away. Judging by how dark the unglazed cobalt decoration is there is a lot of cobalt there. Adding some flux, like a little gerstley borate or frit will help the glaze from crawling, 10% should be enough. The gerstley or frit also helps avoid a dusty oxide layer, which would also contribute to crawling.

Have you tried using watered down underglaze instead of cobalt oxide? Might be worth a try.

I noticed your glaze is really crazed, is this an issue? One last thing, glazing just one side of a pot can cause dunting/cracking issues especially with functional ware. Nice line quality with your technique.

[Magnolia Mud Research]

6 hours ago, Guillaume said:

What I used is 5% cobalt oxide mixed with ... water based acrylic paint. I did that because I wanted to fill an empty marker and draw (I don't like to work with brushes). I thought that it might be a good medium + the paint will burn away. 

I have used watercolors on bisque ware followed by  glazes.  Treat both as if you were making a watercolor painting.  adding some soluble organic material that will act as glue when dried can also help: skimmed milk, sugar, clear water color medium, ... 

I have used clear acrylic and cobalt blue acrylic paint on both greenware and bisque ware and the acrylic does act as a repellant to water based glaze.  Mixing the clear glaze into a clear acrylic would probably work, but why go that route? 

cobalt acrylic paint does work on both greenware and bisque, but the bonding is poor unless something similar to baking soda added to the "paint' to assist in the low temperature bonding when firing. 

Keep tinkering with the materials.

LT

[Guillaume]

Thanks a lot for your answers, and sorry for not giving all the details first. So the drawings were made on bisque. I don’t think I could do it properly on green ware as it might get “wet / soft” ? Then I added the transparent glaze and fired it. 

To Min: I also thought that I put too much oxide, or squeezed the marker too much now and then and have those dark unglazed moment … because sometimes the line is good. There are some blue moment here and there!

So, from what I (can) understand … would it be possible to make the drawing, then bisque it again to burn the acrylic, then glaze and fire it properly ? Or do I have to find a replacement to the acrylic ? Or do I use underglaze instead (not familiar with this at all) ?

I can make a test with just water and oxide and see if it run through the pen properly but I got the feeling that without a smooth medium it might not work, but again, I base those thoughts on my illustrator experience and it might be totally different here. 

Any thoughts ?
Thanks. G. 

[Min]

I think it's great you are thinking outside the box insofar as application method. If the same line quality is what you are looking for I'd make up some test tiles and try using your same method of acrylic paint plus cobalt oxide and give it a try on leatherhard greenware, bone dry greenware and bisque but run them all through a bisque before glazing them. (for the tests on bisque you probably only need to heat to about  500C to burn off the organics since they have already been bisqued but it won't hurt to re-bisque fire them) Give them a good wiping down with a damp sponge prior to glazing to get any ash dust off from burning the paint off. (dust can cause glaze crawling too) If you have some gerstley borate or frit try the same tests with approx 10% of that added and see what happens. Keep good track of what you are doing so you can replicate the successes and use a good scale to weigh out the cobalt etc. 

I don't know what underglazes are available to you in Germany. On this side of the pond they come in a thick slip like consistency that can be watered down and applied by spray, brush, stamp, brush etc. In the EU I believe there are also underglaze powders which are mixed with an underglaze medium and used similarly.  They can be applied at leatherhard, dry greenware or bisque. A second firing isn't usually necessary prior to glazing as long as the glaze doesn't smudge the underglaze. It would be interesting to see if they could be applied in the same way as you've been doing. 

 

[Babs]

Glycerin or cmc would make it painterly.

[Guillaume]

So … Homework

 

Mix 1 : 5% Cobalt Oxide + 95% Acrylic paint
— on leather hard green ware
— on bone dry green ware
— on bisque

Mix 2 : 5% Cobalt Oxide + 10% frit + 85% Acrylic paint
— on leather hard green ware
— on bone dry green ware
— on bisque

Mix 3 : underglaze, watered down if slip OR powder + medium (ratio?)
— on leather hard green ware
— on bone dry green ware
— on bisque

Note : I don’t get the difference trying this on leather hard and bone ?!?

 

Mix 4 : 5% Cobalt Oxide + 95?% Glycerin + water ?
— on bisque

Note : is CMC sugar ? just googled it … ?!?

 

Will let you know how it turns out 
If anybody got another idea, please let me know
Thanks to all for the effort ! G.

[Babs]

If you put it onto bisque and do not fiire prior to glazing you will get the exact same results i.e. glaze repelled by acryllic paint.

CMC mix it in cold water then cook it

[PeterH]

12 hours ago, Babs said:

Glycerin or cmc would make it painterly.

Genuine question, does this still apply if it's applied by a [felt-tip?] marker.

PS @Guillaume Could you post a picture of both your marker pen and its tip, so we can be certain what you are using?

[PeterH]

35 minutes ago, Guillaume said:

 Note : is CMC sugar ? just googled it … ?!?

CMC (carboxymethyl cellulose) is a gum, sometimes used in wallpaper pastes. https://digitalfire.com/material/cmc+gum

... sold by some pottery stores and readily available from the UK ebay  (just looked, now mainly under the trade-name Tylose).

[Babs]

11 hours ago, PeterH said:

Genuine question, does this still apply if it's applied by a [felt-tip?] marker.

PS @Guillaume Could you post a picture of both your marker pen and its tip, so we can be certain what you are using?

Not sure at all re felt tip but can't see why not as the felt is only the carrier.