Creating a white-on-black oil spot glaze effect at cone 6 oxidation

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See Also:
Three safe, reliable and durable cone 6 jet black glazes


Project Name

Simulated Oil Spot Effect (white over black)

Project Codenumber

UnAssigned

Notes

Challenges to creating a layered oil-spot glaze effect:

Choose a good black. Theoretically it needs to be melt-fluid and mobile. In this project I compared G3914A, G2926BB and G2934BL. But only one of them worked, the Alberta Slip version. At first I thought this was because it has a higher LOI. But it does not, it is only 3.5%. Then I thought it was because it has more coarser particles (because of the Alberta Slip), but after ball milling it continued to work?

Choose a good white: Theoretically, it needs to have a higher surface tension. Yet, as you will see in the notes for G3912A, I mistakenly ended up with one of low surface tension. Yet it still worked!

So I am going to do more work on this to try to understand why the G3912A/G3914A combination works and the others do not.

Pictures

The effect I am seeking

The surface tension on a white glaze floating on a black under glaze pulls it into islands revealing the color below.

Surface Tension White Tin

Code #

G3912A

Materials Amt
Strontium Carbonate 23.529
Ferro Frit 3195 15.294
Ferro Frit 3110 15.294
EPK 20.000
Silica 25.882
Additions
Tin Oxide 6.000

Total:106.00

Auto Unity Formula

CaO 0.18
K2O 0.02
Na2O 0.20
(KNaO) 0.21
SrO 0.60
B2O3 0.21
Al2O3 0.37
SiO2 3.33
SnO2 0.15

Ratios

Si:Al: 9.1:1
SiB:Al: 9.7:1
R2O:RO: 0.2:0.8

Expansion

7.0 (Molar:6.7)

LOI

10.0

Cost

0.00 per kg

Notes

*This was calculated from G2934, replacing CaO and MgO for SrO, to increase the surface tension of the melt and add LOI to create bubble turbulence. I later realized the SrO would actually reduce the surface tension (high MgO is the key to high surface tension so I should have just used the original G2934Y recipe with added zircopax). Yet this worked anyway in my simulated oil-spot glaze project.

The change has produced a non-typical chemistry (0.60 SrO would be highly unusual). However it does behave similarly to CaO (and high-CaO is very normal in glazes). However this recipe sources plenty of Al2O3 and SiO2 and it is melting very well, so there is no reason to believe it would leach.

We tried 4,5,6,7% tin. There appeared to be no benefit to going beyond 6.

Initial mix was 400 water and 500 powder. It was too thin but epsom salts gelled it.

Pictures

G3912A with 4,5,6,7% tin oxide

Fired cone 6 PLDSSC firing.

Cone 6 V-359 engobe, G3912A glaze

At cone 6 this is more fluid than expected. However there is plenty of bubbling from the strontium, this is beneficial to the oil-spot effect.

Which black base layer produces the best oil-spot effect?

G3912A on G2934Y, G2926BL, G3914A blacks at cone 6. Clearly this white is sensitive to the black glaze it is on. It produces the best oil spot effect on G3914A.

Untitled

G3914A black with G3912A overglaze - 4-7% tin

The first black one (with 4% tin overglaze), only has 3% black stain.

G3912A on G3914A at cone 8

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Surface Tension White Tin" id="186948" key="EM2dzCSi" date="2021-03-19" codenum="G3912A"> <recipelines> <recipeline material="Strontium Carbonate" amount="23.529" tolerance=""/> <recipeline material="Ferro Frit 3195" amount="15.294" tolerance=""/> <recipeline material="Ferro Frit 3110" amount="15.294" tolerance=""/> <recipeline material="EPK" amount="20.000" tolerance=""/> <recipeline material="Silica" amount="25.882" tolerance=""/> <recipeline material="Tin Oxide" amount="6.000" added="true"/> </recipelines> </recipe> </recipes>

Born: 2018-01-29, Modified: 2021-03-19 09:03:55

Alberta Slip Gloss Black

Code #

G3914A

Materials Amt
Alberta Slip 40.000 38.46%
Alberta Slip 1000F Roasted 40.000 38.46%
Ferro Frit 3195 20.000 19.23%
*Mason 6600 Black 4.000 3.85%

Total:104.00

Auto Unity Formula

CaO 0.45
MgO 0.28
K2O 0.11
Na2O 0.17
(KNaO) 0.27
TiO2 0.01
B2O3 0.23
Al2O3 0.51
SiO2 3.15
Fe2O3 0.08

Ratios

Si:Al: 6.1:1
SiB:Al: 6.6:1
R2O:RO: 0.3:0.7

Expansion

7.2 (Molar:6.7)

LOI

4.7

Notes

*This black employs the proven Alberta Slip GA6-B base. This produces a slightly variegated surface (it glistens in the light). Since Alberta Slip is a natural material and contains significant iron already, a jet-black glaze is produced with only a 4% stain addition!

This glaze really benefits from slow-cool firing (e.g. the C6DHSC schedule), giving a much more glassy surface (especially important with a glossy black).

Making this recipe yourself is doable, but made more difficult by the fact that some of the Alberta Slip must be roasted and all of it must be ball milled to remove coarser particles.

For use as a traditional single coat dipping glaze: Mix this into water at about equal weights of powder:water to produce a creamy slurry.

For a base-coat dipping version, a CMC gum addition is needed. We have found that a 5:95 Laguna Gum Solution:Water ratio produces a slurry that still dries reasonably fast (add 113 powder to 100 gumsol/water mix). Add 0.35% CMC powder to the glaze recipe and blender mix 113 powder to each 100 water. If added coats pull this away, more gum is needed (with longer drying time).

To make a 500ml jar of brushing glaze we mix 340 powder, 440 water, 5 Veegum (1.8%), 5 CMC gum. Blender mixing is required to mix it, other mixers will not be able to wet gum particles and create the gelled consistency needed for painting. On a larger scale, 5000 grams powder will make 15 500 ml jars (2 US gallons). These proportions produce a slurry having 40% solids, 60% water and 1.31 specific gravity.

Pictures

Three cone 6 black glazes

Left: 2926B black (mislabelled as 2917F)
G3914A Alberta Slip black.
G2934 satin black.

The G3914A (#2) has surface variegation that the other two lack (a combination of healed micro-bubbles and precipitation of the black). While the 4% stain might be a little too much, this did survive and overnight lemon-juice test without any visible effect.

G3914A 3% stain with G3912A - 6% tin

G3914A black with G3912A overglaze - 4-7% tin

The first black one (with 4% tin overglaze), only has 3% black stain.

Which black base layer produces the best oil-spot effect?

G3912A on G2934Y, G2926BL, G3914A blacks at cone 6. Clearly this white is sensitive to the black glaze it is on. It produces the best oil spot effect on G3914A.

G3914A Black at cone 6 on M340

While only having 4% stain, the surface has a unique metallic and crystalline quality. The full effect requires ball milling.

G2926B Black vs. G3914A Black

Left half: G2926B with 6% Mason 6600 stain.
Right half: G3914A is Alberta Slip base GA6-B with 4% Mason 6600 stain added.
The latter has a fine crystalline character that is much more interesting.

G3914A on Coffee clay and M370 at cone 6

PC-20 over 14A (left) and C1 (right)

The PC-20 is thinning the G2914A but it is bluer.

G3914A vs Amaco C-1 Obsidian Black

Needs more stain to equal the opacity. However this glaze features a microcrystallizing surface, especially on slow cooling, it is not likely a good candidate for more stain. Better to use the G2926B black.

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="Alberta Slip Gloss Black" id="188926" key="wi4Tj7he" date="2023-06-23" codenum="G3914A"> <recipelines> <recipeline material="Alberta Slip" amount="40.000" tolerance=""/> <recipeline material="Alberta Slip 1000F Roasted" amount="40.000" tolerance=""/> <recipeline material="Ferro Frit 3195" amount="20.000" tolerance=""/> <recipeline material="Mason 6600 Black" amount="4.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2020-11-17, Modified: 2023-06-23 11:48:54

G2926B Cone 6 Gloss Black

Code #

G2926BL

Materials Amt
*G2926B Clear Glaze 100.000 94.34%
*Mason 6600 Black Stain 6.000 5.66%

Total:106.00

Notes

*This is based on G2926B, our standard transparent glaze for cone 6. We have made this for many years and are confident of its durability and safety (it has the maximum SiO2/Al2O3 possible for the gloss and is made from safe materials sourcing safe oxides). As of 2021 we are beginning to use 45 micron silica and Fusion Frit F-12 instead of Ferro 3134). Both of these changes appear to have improved the glaze, making it more melt fluid and of lower thermal expansion (likely because the silica is going into solution in the melt better).

It fires jet black super gloss. Compared to common methods of making cone 6 blacks (using up to 20% raw metal oxides), this should be much safer to use and much less likely to be leachable. 6% stain is a very low percentage, we are confident that this would be OK on food surfaces (as confirmed by our GLLE testing). The character of the black is glassy and homogeneous, so it may be too industrial-looking for many potters.

For 500ml jar (1 pint) blender mix 440 water, 330g powder, 5 CMC gum and 5 VeeGum.

Pictures

G2926B Black vs. Low Fire black

While G2926B Black looks impressive at first, when you hold it beside a low temperature glossy black (like Spectrum one) it is not as good. Ball milling would improve that.

A stunning contrast between matte and gloss blacks

The clay is Plainsman M370. Fired at cone 6 using the PLC6DS drop-and-hold firing schedule. The inside glossy glaze is G2926BL. The outside glaze base is G2934BL matte. Both recipes contain 6% Mason 6600 black stain. G2934BL is tricky to keep consistent because the matte surface is a product of both the chemistry and the firing schedule. Thus we faced lots of testing when it became necessary to substitute Ferro Frit 3124 for the supposed equivalent, Fusion Frit F-19. Early results showed a little better melting, so the 10-15% glossy we normally add to move the stoney matte toward satin is not needed. However, we still made an 85:15 batch for our more frequent slow-cool C6DHSC firings (otherwise this G2934 mug would have fired too matte). So with the two recipes and two schedules I can produce four surfaces, from gloss satin to stony matte.

Which black base layer produces the best oil-spot effect?

G3912A on G2934Y, G2926BL, G3914A blacks at cone 6. Clearly this white is sensitive to the black glaze it is on. It produces the best oil spot effect on G3914A.

G2926B Black vs. G3914A Black

Left half: G2926B with 6% Mason 6600 stain.
Right half: G3914A is Alberta Slip base GA6-B with 4% Mason 6600 stain added.
The latter has a fine crystalline character that is much more interesting.

Amaco Obsidian vs G2926BL

The obsidian is bluer, more transparent, not as black.

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="G2926B Cone 6 Gloss Black" id="191019" key="ofRJPuHc" date="2023-06-27" codenum="G2926BL"> <recipelines> <recipeline material="G2926B Clear Glaze" amount="100.000" tolerance=""/> <recipeline material="Mason 6600 Black Stain" amount="6.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2021-02-03, Modified: 2023-06-27 13:04:42

G2934 85:15 Adjustable Matte Black

Code #

G2934BL

Materials Amt
G2934 85.000 80.19%
G2926B 15.000 14.15%
*Mason 6600 Black Stain 6.000 5.66%

Total:106.00

Notes

*Attractive black matte glazes are among the most difficult of all effects to produce. Even slight differences in the degree of matteness are noticeable. Cutlery marking is very visible when the surface is too matte. The key to success is adjustability, both in recipe and firing schedule (more info below).

G2934 has long been our favourite matte recipe for cone 6. While it is OK on porcelains and in firings that cool quickly, is often too matte in firings that cool slowly (like heavily packed kilns). For bight colored versions of this base (e.g. orange, yellow, red) we have used the G2934Y base, it develops a more satin surface. But for black that does not work, it often fires too glossy, something you do not want.

Thus we add enough of the G2926BL glossy to tune the degree of matteness. In our kilns, an 85:15 blend works the best. If that fires too matte try 80:20. It might be best to mix a batch of both G2926BL and G2934BL, then pour them together in the desired proportion. Adjust according to the fired result.

Pictures

A stunning contrast between matte and gloss blacks

The clay is Plainsman M370. Fired at cone 6 using the PLC6DS drop-and-hold firing schedule. The inside glossy glaze is G2926BL. The outside glaze base is G2934BL matte. Both recipes contain 6% Mason 6600 black stain. G2934BL is tricky to keep consistent because the matte surface is a product of both the chemistry and the firing schedule. Thus we faced lots of testing when it became necessary to substitute Ferro Frit 3124 for the supposed equivalent, Fusion Frit F-19. Early results showed a little better melting, so the 10-15% glossy we normally add to move the stoney matte toward satin is not needed. However, we still made an 85:15 batch for our more frequent slow-cool C6DHSC firings (otherwise this G2934 mug would have fired too matte). So with the two recipes and two schedules I can produce four surfaces, from gloss satin to stony matte.

Which black base layer produces the best oil-spot effect?

G3912A on G2934Y, G2926BL, G3914A blacks at cone 6. Clearly this white is sensitive to the black glaze it is on. It produces the best oil spot effect on G3914A.

Black matte mixed July 2021

Starting in June 2021 we are switching to Fusion Frits. This batch was made using Fusion Frit F-19 and 45 micron silica. The upper pail is 85:15 Matte/Glossy mix, the lower one G2934 only.

G2934 black vs. G2934:G2926B blend black

Left is G2934 with 6% Mason 6600. Too matte for most people. Right is 85:15 G2934:G2926B with 6% stain. In this C6DHSC firing is has the degree of matteness that we like.

G2934Y and G2934 Black - Fast cool

If G2934Y is slow cooled it looks like G2934 fast cooled.

Untitled

G2934BL brush on version

Four coats. Works very well.

XML (to paste into Insight)

<?xml version="1.0"?> <recipes version="1.0" encoding="UTF-8"> <recipe name="G2934 85:15 Adjustable Matte Black" id="191021" key="hWu58X19" date="2022-10-24" codenum="G2934BL"> <recipelines> <recipeline material="G2934" amount="85.000" tolerance=""/> <recipeline material="G2926B" amount="15.000" tolerance=""/> <recipeline material="Mason 6600 Black Stain" amount="6.000" tolerance=""/> </recipelines> </recipe> </recipes>

Born: 2021-02-03, Modified: 2022-10-24 17:10:31