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G3806C - Panama Cone 6 Adjustment 2015

Modified: 2024-07-24 17:59:11

High fluid melt glaze for reactive effects and super gloss colors

Notes

This is work I did in 2015 (in 2019 a much bigger project developed this further).

The copper and tin produce the turquoise celadon effect.

This recipe is for a brilliant fluid-melt transparent base glaze, initially for copper blues and greens, but later for stains. "Fluid-melt" means it runs down off ware if applied too thickly, this is a key for achieving many visual effects.

Initiailly I compared a number of recipes I found on line and finally selected Panama Blue. I removed the colorants and made adjustments to improve slurry properties and lower the thermal expansion (it has serious crazing issues). Fluid-melts have a down side: Crazing is an issue (because the fluid melt requires more fluxes, these have higher thermal expansions).

Then I did three adjustments, each lowering the thermal expansion more than the last. While keeping the same brilliant visual appearance. The recipe ended up being quite different (two materials were eliminated from the recipe, their oxides supplied by the others). The chemistry of this one moves much of the KNaO to low-expansion MgO. This makes it melt a little less, but visually it is the same. Higher ZnO helps melting (since MgO is not nearly as powerful a flux as KNaO). I was even able to add extra SiO₂. The calculated thermal expansion has gone from 7.7 down to 7.3.

This worked well on stonewares but still crazed on Plainsman P300 and M370 (but was OK on Polar Ice). Fluid melt glazes look best on porcelains so this was obviously a problem. So I continued development in pursuit of a fluid melt having a lower thermal expansion (see subsequent articles, recipes and posts).

Related Information

Why this copper glaze does not micro-bubble or craze:

High cone 6 melt fluidity, low surface tension, MgO

This picture has its own page with more detail, click here to see it.

This green is not just a typical transparent cone 6 glaze with 2% copper carbonate added (and 2.5% tin oxide). That outer glossy glaze accommodates the copper without micro-bubbling or crazing because of its lower melt surface tension. In such glazes, significant MgO (a super low expansion oxide) can often be tolerated without losing gloss. This is a light bulb moment. Fully 0.15 molar of MgO are present here. This is the "matting oxide"! Yet the glaze is still hyper-glossy!

The above factors are enough. But if this were used in industry, technicians would fix additional issues: The very low initial melting temperature (from 37% very early-melting frit in the recipe). That traps LOI bubbles unnecessarily. Raising the ZnO and sourcing as much of the B₂O₃ and KNaO as possible from later melting materials and/or frits.

The porcelains are Plainsman P300 and M370. The liner glaze is G2926B, it is a gloss but has a much lower melt fluidity, it is a functional transparent whose main job is to fit the body and be hard and durable. The outer glaze is G3806C.

More copper can produce fewer bubbles!

This picture has its own page with more detail, click here to see it.

The first glaze, G2926B, is a standard functional transparent glaze with added copper. The other three are part of a project to find a copper blue (L3806B has the best color and the best compromise of flow and bubble-clearing ability).

The GLFL testers for melt flow at the back, and the GBMF test melt-down-balls contain 1% copper carbonate. The glazed samples in front have 2% copper carbonate. But why do the recipes containing half the amount of copper have far more bubbles? Because they are thinner? Not really, in use on ware, they also have fewer bubbles. Why? A small CuO addition can change where and how bubbles nucleate and how viscous the melt is. At some point between 1% and 2%, a threshold is crossed that affects nucleation and coalescence. For example, a little copper could encourage lots of tiny bubbles to form and stay trapped, while more changes the melt chemistry enough that they coalesce and escape (or simply aren’t nucleated the same way). Phase separation could produce Cu-rich droplets that enable copper to be its own fining agent.

Our G2926B glaze may not work on dark burning clays

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These two glazes, applied to the outsides of these mugs, both fire as brilliant glass-like super-transparents. But on this high-iron stoneware, from which both pieces are made, only one is working well. G3806C (on the outside of the piece on the left) melts more, it is fluid and much more runny. This melt fluidity gives it the capacity to pass the micro-bubbles generated as the body gases during firing. G2926B (right) works great on porcelain and buff stoneware but it cannot clear the clouds of bubbles coming out of this body (the bubbles are actually partially opacifying it). Even the normal glassy smooth surface has been affected. The moral: Potters need more than one base transparent recipe. Being able to host colors, opacifiers and variegators is nice, but sometimes just a transparent that works well is needed. An interesting trade-off of reactive melt-fluid glazes is that, while they develop more interesting surfaces, their lower SiO₂ and Al₂O₃ contents make them susceptible to crazing, settling of the slurry and cutlery marking.

An ultra-clear brilliantly-glossy cone 6 clear base glaze? Yes!

This picture has its own page with more detail, click here to see it.

I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram GBMF test balls of each melted down onto a tile to demonstrate melt fluidity and bubble populations. Second, third, fourth rows show them on porcelain, buff, brown stonewares. The first column is a typical cone 6 boron-fluxed clear. The others add strontium, lithium and zinc or super-size the boron. They have more glassy smooth surfaces, less bubbles and would should give brilliant colors and reactive visual effects. The cost? They settle, crack, dust, gel, run during firing, craze or risk leaching. Out of this work came the G3806E and G3806F.

Almost final recipe for cone 6 copper blue - G3806B

This picture has its own page with more detail, click here to see it.

This is the winner of a five-way cone 6 copper blue glaze comparison that started with my dissatisfaction with Panama blue. When I compared these glazes, I did not just eyeball them on a tile. I compared the melt flow, thermal expansion and slurry performance of the bases (without the copper and tin). Ball-melt GBMF tests also showed bubble and color development for very thick sections. Then I tried more copper and did more flow tests. I also did leaching tests. Where needed I adjusted recipes to increase clay content (while maintaining chemistry) so the slurries would work better. Without my account at insight-live.com to keep all of this organized it would have been so much more difficult; actually, I probably would not even have bothered with the project. The final recipe, G3806C, was an adjustment to this G3806B to reduce the thermal expansion.

A problem with brilliantly colored fluid-melt glazes: Micro-crystals

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Crystallization (also called devritrification). You can see the tiny crystals on the surface of this copper stained cone 6 glaze (G3806C). The preferred orientation of metallic oxides is crystalline. When kilns cool quickly there is simply not enough time for oxides in an average glaze to organize themselves in the preferred way and therefore crystals do not grow. But if the glaze has a fluid melt and it cools slowly through the temperature at which the crystals like to form, they will. There is another issue here also: There are tiny dimples in the surface. This is because copper carbonate was used here instead of copper oxide. During firing, it generates carbon dioxide (because it is a carbonate) that bubbles out of the melt, leaving behind dimples that may or may not heal during cooling.

Underglazes require a fluid melt transparent

But extra melt fluidity comes at a cost

This picture has its own page with more detail, click here to see it.

These porcelain mugs were decorated with the same Amaco Velvet underglazes (applied at leather hard), then bisque fired, dipped in clear glaze and fired to cone 6 (using a drop-and-hold schedule). While the G2926B clear glaze (left, A) is a good glossy transparent for general use, its melt fluidity is not enough to clear the LOI micro-bubble clouds that dull the colors below (strangely, underglaze colors can generate them too). However, the G3806C recipe (right, B) has a more fluid melt, which is one factor that better enables bubble escape. Its melt also has a lower surface tension. An additional factor is thickness. A thinner layer of B would be even better (brushing glazes enable thin layering).

But B has downsides. Running is one, but not a factor here because we want it thin. Its high flux content also means it fires less durable. And, its high KNaO content raises the thermal expansion (COE) and thus the danger of crazing. Although it fits this porcelain, it crazes on others.

In pursuit of a reactive cone 6 base that I can live with

This picture has its own page with more detail, click here to see it.

These GLFL tests and GBMF tests for melt-flow compare 6 unconventionally fluxed glazes with a traditional cone 6 moderately boron fluxed (+soda/calcia/magnesia) base (far left Plainsman G2926B). The objective is to achieve higher melt fluidity for a more brilliant surface and for more reactive response with colorant and variegator additions (with awareness of downsides of this). Classified by most active fluxes they are:
G3814 - Moderate zinc, no boron
G2938 - High-soda+lithia+strontium
G3808 - High boron+soda (Gerstley Borate based)
G3808A - 3808 chemistry sourced from frits
G3813 - Boron+zinc+lithia
G3806B - Soda+zinc+strontium+boron (mixed oxide effect)
This series of tests was done to choose a recipe, that while more fluid, will have a minimum of the problems associated with such (e.g. crazing, blistering, low run volatility, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually evolved the G3806B, after many iterations settled on G3806E or G3806F as best for now.

2% Copper carbonate in two cone 6 transparents:

One does not bubble and orange-peel. Why?

This picture has its own page with more detail, click here to see it.

The top base glaze, G2926B, has enough melt fluidity to produce a brilliant functional gloss when used as a transparent. However, for this 2% copper carbonate addition, it has too little melt fluidity and/or too much surface tension to merge, pass and heal the entrained micro-bubbles (generated by the decomposition of the carbonate).

The lower glaze, G3806B, diversifies the fluxes (half the B₂O₃ in exchange for more Na₂O and introduction of SrO and ZnO) and increases their total compared to Al₂O₃ and SiO₂. The result is a more fluid cone 6 melt having lower surface tension. The mixed-oxide effect is also a factor here; the diversity itself improves the melt.

The above factors are enough to solve the problem here. But more can be done. More zinc (in exchange for KNaO) could produce later melting, especially in combination with sourcing some or all of the latter from a feldspar instead of the low-melting frit. The benefit would enable more gas escape until melt-sealing (and reduce the COE).

Two base clear glazes with 2% copper:

One is bubbling and one is not.

This picture has its own page with more detail, click here to see it.

By itself, without copper, the G2926B recipe (right) produces a better and more durable glass (comparing the cups in the back). But a 2% copper addition, front, turns its surface to a mass of unhealed bubble-escapes. The G3808A recipe, on the left, develops much more melt fluidity, the extra mobility enables the bubbles, created by the decomposing copper, to coalesce, grow, break at the surface and heal before the melt stiffens too much.

Copper oxide needs a fluid-melt transparent to produce a glossy glaze

This picture has its own page with more detail, click here to see it.

Fired at cone 6. A melt fluidity comparison (behind) shows the G3808A clear base is much more fluid. While G2926B is a very good crystal clear transparent by itself (and with some colorants), with 2% added copper oxide it is unable to heal all the surface defects (caused by the escaping gases as the copper decomposes). The G3808A, by itself, is too fluid (to the point it will run down off the ware onto the shelf during firing). But that fluidity is needed to develop the copper blue effect (actually, this one is a little more fluid that it needs to be). Because copper blue and green glazes need fluid bases, strategies are needed to avoid them running off the ware. That normally involves thinner application, use on more horizontal surfaces or away from the lower parts of verticals.

A fluid melt glaze bleeds much more into adjoining ones

This picture has its own page with more detail, click here to see it.

The outer green glaze on these cone 6 porcelain mugs has a high melt fluidity. The liner on the upper mug is G3806C, a fluid melt high gloss clear. The liner glaze on the lower one, G2926B, is high gloss but not highly melt fluid. Thus, when both the outer and inner glazes have high melt fluidity (upper mug), they bleed together forming a fuzzy boundary. But when even one of them is not, a crisp boundary is achieved (lower mug).

G3806D cone 6 glaze with copper carbonate, copper oxide

This picture has its own page with more detail, click here to see it.

This G3806D fluid melt glaze recipe demonstates the different color characteristics imparted by copper carbonate (left) and copper oxide (at 2%). The carbonate version is bluer and less intense. Copper carbonate is about 65% CuO while the oxide version is 100%. Our supply of the oxide version is not producing any specking (if yours does you may need to sieve or blender mix the slurry).

Why do these cone 04 and 6 clear glazes have so similar a chemistry?

This picture has its own page with more detail, click here to see it.

The glaze on the left (as shown in my account at insight-live.com) is a crystal clear at cone 04. The high frit content minimizes micro-bubbles. The high B₂O₃ melts it very well (this has 0.66 B₂O₃, that is three times as high as a typical cone 6 glaze). The recipe on the right is the product of a project to develop a low-thermal-expansion fluid-melt transparent for cone 6 (with added colorants fluid melts produce brilliant and even metallic results and they variegate well). While the balance of fluxes (the red numbers in the formula) is pretty different, look how similar the B₂O₃, Al₂O₃ and SiO₂ levels are (yellow, red and blue backgrounded numbers in the formula), these mainly determine the melting range. That means that a fluid-melt cone 6 glaze may actually be just a low temperature glaze being overfired to cone 6.

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XML to Paste Into Insight-live

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<recipe name="Panama Cone 6 Adjustment 2015" keywords="High fluid melt glaze for reactive effects and super gloss colors" id="125" date="2024-07-24" codenum="G3806C">
<recipelines>
<recipeline material="Silica" amount="26.300"/>
<recipeline material="Kaolin" amount="19.700"/>
<recipeline material="Dolomite" amount="8.700"/>
<recipeline material="Strontium Carbonate" amount="4.400"/>
<recipeline material="Ferro Frit 3110" amount="31.100"/>
<recipeline material="Ferro Frit 3134" amount="6.600"/>
<recipeline material="Zinc Oxide" amount="3.300"/>
<recipeline material="Copper Oxide" amount="2.000" added="1"/>
<recipeline material="Tin Oxide" amount="2.500" added="1"/>
<url url="https://digitalfire.com/recipe/125" descrip="https://digitalfire.com/recipe/125"/>
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