I'm wanting to create some blue glazes using copper carbonate, I'm currently working from the digital fire cone 6 base glazes. From what I have read I need to reduce the alumina and increase the alkali to get blues. Can anyone suggest what percentage the alumina, soda and potash should be at, because my calculations so far have led to very low melts, and very low expansion with very high gloss... I am planning to use cobalt as a colourant but would like to see if a copper blue is possible for me. Thanks !

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I recently added this recipe to the Insight-Live database.

Digitalfire colors: http://digitalfire.com/4sight/properties/ceramic_property_glaze_col...

"Crystal Emerald Green" which becomes "Crystal Emerald Blue" by merely reducing the percentage of Copper Carbonate from 4% to 0.5%.

Although this glaze is relatively low in Alumina and alkaline with a lot of calcium, I suspect the Lithium Carbonate may be part of the reason the copper shifts to a blue shade at a low percentage.

Thanks Norm but I'm a bit confused by your photo, are the two on the left the same glaze on different clay?

This ^6 alkaline glaze using only 0.5% copper carbonate reflects a blue color.

On the left is the glaze on white "Laguna Greystone clay" and in the middle on "Laguna WC-373 Dark Brown", both fired to Cone 6. 

On the far right is the same glaze on the Dark Brown clay fired to just ^04. I usually fire a tile of new glaze in a bisque to determine how the glaze melts by that temperature, how mobile it is and whether it would make an interesting Bisque temperature glaze.  It makes a pretty nice bisque temperature glaze.

You can see that very high level of Soda Feldspar has left the glaze crazed when fired to Cone 6. The Lithium Oxide in the lithium carbonate is also an alkaline oxide which also makes the low level of copper blue. The lithium oxide also leads to lower expansion which offsets some of the high expansion imparted by the soda feldspar.

The crazing is not present when you make the Crystal Emerald Green recipe, with the full 4% of copper carbonate fired to Cone 6 (below), probably because the copper acts as additional flux.

I think the concept of a glaze which is Alkaline or Acidic can be misleading to most. 

It doesn't refer to the pH at room temperature, which depends on the net amount of "proton donors" which are acids, and "proton acceptors" which are alkalies.

I was going to send you this in an email, but I might as well post it.

The acidity or alkalinity of a glaze when melted is calculated by how many metal atoms there are in ratio to the number of oxygen molecules. When molten, silica sand SiO2 is acidic because there are two oxygen atoms to each silicon atom. This is a pattern of RO2, where R is any metal and O is oxygen.

More to the point, silica has been observed by chemist reacting with oxides which they call alkaline, so silica has to be an acid. Fair enough.

When I first read that molten sand is acidic, I understood from taking chemistry at U.C. Berkeley where this explanation was going.  But to my every day world it doesn't even sound plausible or have any real world meaning.  How would you test the acidity of a liquid rock which is 2,200 F or 4,000 F?  Their answer is you test it with an oxide they call alkaline an see if it reacts.  There you go, asked and answered.

So I don't say this from personal first hand knowledge. These patterns and the behavior of how each oxide reacts with other oxides has been observed by chemists and assigned to the category of acid, base, neutral, or amphoteric. And I'll have to take their word for it.

These are some of the most commonly used oxides and their acidity or alkalinity - based on observation by chemists.

Aklaine Fluxing Oxides R2O and RO
AG2O silver
K2O potassium
Li2O lithium
Na2O sodium
+
BaO barium
CaO calcium
CuO copper
FeO iron
MgO magnesium
SrO strontium
Neutral Refractory Oxides R2O3
Al2O3 alumina
Bi2O3
bismuth
Both Refractory and Amphoteric
CrO3
B2O3

chrome

boron

also called a glass-former" but it's amphoteric or neutral depending on the use.

 

Amphoteric -   RO and R3O4
Basic to acids, Acidic to bases
PbO lead
Pb3O4 lead
SnO tin
ZnO zinc

Acidic Oxides

Some call these glass-formers, primarily referring to Silica and Zircon. Other acidic Oxides don't really fit the term "glass former."

MnO2 manganese
SiO2 silica
TiO2 titanium
ZrO2 zircon
CO2 carbon
NO2 nitrogen
NO3 nitrogen
SO2 sulfur
SO3

sulfur

You see the oxygen to metal atom ratio tends to predict what class each oxide fall into, but there are many exceptions and I'm certain there are elegant explanations for those exceptions which would give me a headache.

I have read that the oxides of Boron, Silicon, Germanium and Phosphorus are the only oxides which can form a single oxide glass.  But Sapphire is almost pure alumina glass, naturally occurring sapphire has impurities. Man-made alumina glass requires the introduction of "Rare Earth elements."  http://physicsworld.com/cws/article/news/2004/aug/11/glass-breakthr...

This is but a small sample of what's involved explaining one subset of oxides, focused on electron transfers, just as room temperature acids and bases transfer protons.

http://www.chemguide.co.uk/inorganic/group4/oxides.html

Chrome Oxide is in the R2O3 pattern of a neutral refractory oxide, but from observation by chemists of the results produced Chrome Oxide also acts like an amphoteric oxide (Greek for Both - in this context both an acid and a base). An amphoteric molecule reacts with a base like and acid and reacts with with an acid like a base. But the amphoteric oxide pattern should be a ratio of RO or R3O4 - so it's another exception.

For that matter some RO pattern oxides are alkaline while others are Amphoteric.  Unless someone can provide me new insights, i've concluded this merely has to be memorized or stored on an Excel sheet as I have done.

http://ceramicsweb.org/articles/glaze_tech/three_oxide_groups.html

Tom - Looking at the Glaze Color page on Digitalfire I see the term alkaline glaze holds no consistent meaning, almost to the extent where it means nothing at all.  Maybe it meant something specific at one point in time, but today it doesn't.

http://digitalfire.com/4sight/properties/ceramic_property_glaze_col...

Personally I use lithium or barium to change the color of copper to blue.

The conflicts are obvious.

1.)  "Oxidation copper blues work best in high alkaline, low alumina glazes. Increasing copper to 4-6% will move color toward turquoise."

Potassium is an alkaline oxide, but it often doesn't work:

2.)  "Potassium K2O can turn a copper glaze yellowish. If Na2O or PbO are present, K2O should not exceed 0.15 equivalent."

Magnesium MgO and Calcium CaO are alkaline oxides, but they don't work"::

3.)  "Copper in calcium / magnesium glazes gives a green very different from that produced with lead."

Lithium Oxide LiO is an alkaline oxide which actually makes copper fire blue:

4.)  "Lithia can produce blue effects with copper."

Barium BaO and Strontium SrO oxides are alkaline and do work:

5.)  "Barium glazes are well known for their ability to produce matte turquoise colors with copper. While strontium is often used to duplicate the matte texture of barium it does not have the same color response."  Barium BaO and Strontium SrO are also alkaline oxides which neutralize the acidity of Silica.

Here's some specific information which doesn't use a vague term like "alkaline glaze"

6.)  Copper in a barium / zinc / sodium glaze gives a blue. Color can also be enhanced by Lithia. Tin and copper can produce turquoise to robin's egg blue."

7.)  Fluoride, when used with copper, can produce blue green colors. Fluoride isn't an oxide but lithium fluoride certainly acts like an oxide.

8.)  7% copper in glossy oxidation glazes can produce striking metallic green colors.

9.)  "Ferro Frit 3230 with very high alkali and little or no alumina can develop these colors well. However they have very high thermal expansion so it is difficult to get them to fit most bodies"

Ferro Frit 3280.

BaO 2.04% 0.028
K2O 11.10% 0.248
Na2O 18.92% 0.641
ZnO 3.21% 0.083
SiO2 64.71% 2.263

That leaves just Silver, Iron and Copper itself as the remaining common alkaline oxides. You can easily add enough Copper Oxide to neutralize the acidity of Silica, but when you do it doesn't make copper into a blue color, but a saturation metallic.

10.)  Large amounts of copper in a glaze give metallic and even graphite effects.

Aklaine Fluxing Oxides R2O and RO    
AG2O    silver
K2O       potassium
Li2O       lithium
Na2O    sodium
+    
BaO    barium
CaO    calcium
CuO    copper
FeO    iron
MgO   magnesium
SrO    strontium

Five years ago I had no clear idea of what these "color guides' meant by an "alkaline glaze".  Today I'm not certain the people who wrote these guidelines did either.

A really interesting, but completely different topic is Boron.  Although it's viewed mostly as a low-temperature flux, it's most useful role is forming different types of borates which prevent crazing and help glaze fit.  Ceramic scientists still don't fully understand the interactions between Boron and either Calcium or Strontium.

Thanks Norm, the line:

 "Oxidation copper blues work best in high alkaline, low alumina glazes. Increasing copper to 4-6% will move color toward turquoise."

Was pretty much all the info I was going on, I couldn't find many other references to copper blues in oxidation.  Your notes here will be a great use to me, and I'm sure other people interested in the subject.

I currently have some tests in the kiln with a high soda feldspar content. I don't have any lithium at the moment, but I will see how these tests come out and go from there.

In my personal experience I've found Potassium makes colors more vibrant, while sodium causes less color brightness but a large increase in glaze expansion , which you can readily verify by putting the recipe into Insight-Live or other glaze calculation program.

In the crystal emerald recipe the increased expansion caused by the sodium is offset by the low expansion created by the lithium.

If you do purchase Lithium Carbonate, buy the finest powder available. http://www.axner.com/lithium-carbonate-fine.aspx

The regular less expensive grade of lithium carbonate provides less fluxing ability because they don't melt through out the glaze, and no amount of grinding I did made it finer.  The lithium carbonate as coarse as caster sugar causes irregular localized effects.

Although it's more costly, I prefer Lithium Fluoride, which definitely does not off gas fluorine.  Combining Lithium and Fluorine is one of the most exothermic reactions known, so breaking it apart would take far more energy than our kilns produce. Lithium Fluoride does boil off at temperatures above cone 10, but condenses unchanged as LiF.

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