Help me understand what happened on floating blue test tiles.

[milksnake12]

Hello! 
I made John Britts' floating blue 2 from his mid fire book. Turned out really nice. However when I looked at it under a stereo microscope (cause everyone has one of those laying around... right?) I could see some crazing. I couldn't see the crazing just looking at it with the naked eye. I thought I would try to lowering the Na a little and increase so other things that shouldn't craze as much. What really surprised me was the dramatic difference in the color which I was not expecting.  The test tile on the left is the original as it is in John's book, the right is my adjustment. 

Floating Blue 2 with lower na

Material	Amount
Nepheline Syenite	27
Ferro Frit 3134	26
EP Kaolin	20
Silica	16
Gerstley Borate	9
Amtalc-C98 Talc	2
Total base recipe	100
Rutile	4
Bentonite	2
Red Iron Oxide	2
Cobalt Carbonate	1.5
Total	109.5

 

 

John's floating Blue 2

Material	Amount
Nepheline Syenite	35.5
Ferro Frit 3134	24.5
Silica	16
EP Kaolin	15.5
Gerstley Borate	8.5
Total base recipe	100
Rutile	4
Bentonite	2
Red Iron Oxide	2
Cobalt Carbonate	1.5
Total	109.5

I can't seem to figure out what happened to the color. Is anything standing out that I did that would cause that?

 

 

[Min]

Floating Blue is a notoriously fussy glaze. Glaze thickness, firing schedule and recipe all need to be bang on for it to work properly. There are a number of factors that make it such a vexing glaze including phase separation that is always talked about with this glaze. I think that what you've done with your altered version is upset the balance enough to loose the blue colour which is coming only partly from the cobalt. 

Looking at your 2 recipes I see that you have doubled the amount of magnesium in the glaze, this is going to have the effect of stiffening the glaze, I know it's still a fairly low amount but this is a very sensitive glaze and changes make a difference. The silica is a fair bit different and likely enough to throw it off too. You've also lowered the sodium, good if only you wanted to reduce crazing but it's important to the phase separation. 

Overall I don't see that the expansion of the glaze might be reduced enough to rid it of crazing in your version. All that being said there are far easier ways to get the look of a floating blue glaze without using this glaze. Simplest way is to layer 2 glazes, use a base glaze with cobalt and iron then a covering glaze that contains rutile. Play around with different black glazes, start with Ron Roy's licorice and try layering glazes overtop of it that contain rutile (or a combination of rutile and titanium dioxide if you find the rutile alone is making the glaze to tan coloured). Salt pig below is done this way with the covering glaze on the top part of the pot. Another option is to have a look at the Alberta Slip version of rutile blue which uses a black engobe underneath to have the same effect.

[jrgpots]

Did you dip both the same amount?  It kind of looks like the glaze on your second tile was not as thick.  The second tile does have some blue float in the middle just to the right where it ran.  Try  dipping a second time or leaving it in the gaze a little longer.

was the second tile bisqued at the same temp as the first?  If they were fired to bisque differently,   it may change how they take up the glaze

 

[milksnake12]

Thank you for the replies! 

I did fire both in the same temps and tried to dip consistent. 

I guess I didn't realize the base had such an effect on the colorants. I'll give the laying a try.