Iron Glazes and Achieving Red Color in Oxidation

Summary

Red iron oxide (Fe2O3) decomposes into iron monoxide (FeO) above a specific temperature, even in oxidizing environments. The formation of iron red occurs through the surface growth of columnar crystals of Fe2O3. The most vibrant reds can be achieved by a rapid cooling process through the temperature range where black FeO tends to crystallize, typically above 950°C, to around 950°C, where the growth of red Fe2O3 crystals is optimized. Oxygen plays a crucial role in this phase. Holding the temperature at 950°C for approximately one hour ensures maximum coverage of iron crystals, yielding the best red color. Prolonged holds beyond this time lead to a deterioration of the color, trending towards rust brown.

Here’s a valuable excerpt from John Sankey's website, providing detailed guidance on achieving iron reds. Red iron oxide (Fe2O3) decomposes to iron monoxide (FeO) above a specific temperature in oxidizing conditions. The ideal formation of iron red results from the surface growth of columnar crystals of iron. The brightest reds are developed through a swift cool through the higher temperature range where black FeO typically forms, down to about 950°C, where optimal red Fe2O3 crystal growth occurs. Adequate oxygen is essential for this transformation. Sustaining the temperature at 950°C for around one hour promotes optimum iron crystal coverage and the deepest red shade. The color starts to diminish with any holds exceeding this duration, edging toward a rusty brown tone.

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The presence of calcium in an iron glaze influences the quality of the red color; a higher calcium content makes achieving a good red more challenging. Nonetheless, some calcium is necessary for creating red from iron. To achieve a tomato red hue, phosphorus is required; when magnesium is added, the resulting color shifts to a more orange tone. The optimal iron reds in oxidation firing occur with approximately 0.10 molar of CaO, 0.03-0.06 of MgO, 0.01-0.02 of P2O5, and 0.08-0.12 of FeO.

Literature

Reported Colors
Black, tomato red, rust red, and muddy brown, along with shades of green, honey, and rust, are mentioned in various sources. My own testing has confirmed the existence of these nuances. Blue hues have only appeared in crust-like surface components; if crystalline, the sizes are too small to observe even at 100x magnification. Yellow or green colors have not been observed as surface components but are apparent within the body. Reds manifest exclusively as a crystalline surface layer.

Reported colors include black, tomato red, rust red, and muddy brown, along with shades of green, honey, and rust. My research has confirmed all of these variations. Blues only show as crust-like surface features; if they are crystalline, the crystals are too small to identify at 100x magnification. Yellow or green appearances only occur within the body, while reds seem to solely exist as a crystalline surface layer.

Application Thickness
Application thickness is often highlighted as a key factor influencing red iron glaze colors. In my experimentation, I have found that a minimum thickness of approximately 50 μm is necessary.

Cooling Schedule
The cooling process is known to be significant. Refiring to cone 04 is often recommended for enhancing reds. Research by Murrow, published in Ceramics Monthly (September), indicated that shino glazes developed red only below 982°C. Marians (Ceramics Monthly, June/July) found critical temperature holds between 980°C and 870°C during the cooling cycle for red development in iron glazes. In my work, I have confirmed these observations; maintaining a hold at 950°C for one hour during cooling yields the richest reds.

Color Analysis
Murrow and Vandiver, noted in Ceramics Monthly (September), discovered that the red of shino glazes originates from a surface layer of ferric microcrystals about 20 μm thick, with a white glaze underneath. My collaboration with John Stirling indicated that the surface of iron glazes presents iron sesquioxide (Fe2O3), while the deeper layers contain iron monoxide (FeO). Park and Lee (J. Ceram. Soc. Japan 113():161-165) identified that in high magnesia glazes, the red color is attributed to magnesioferrite (MgO·Fe2O3).

Calcia
Cardew (Pioneer Pottery) remarked that alkaline earths must be minimized for an iron red; even 0.2 calcia will shift an iron glaze to brown. He proposed a specialized frit to mitigate this issue. My findings back this up regarding calcia.

Magnesia
Park and Lee (op.cit.) found via X-ray diffraction that magnesia assigns a red color as magnesioferrite (MgO·Fe2O3), with its crystal formation closely related to the presence of whitlockite-type crystals (Ca9(Mg,Fe)(PO4)6(PO3OH)). Phosphorus seems to crystallize as whitlockite at lower temperatures and as magnesioferrite at a range of 900-950°C. Edouard Bastarache (unpublished) posits that dolomite (Ca,Mg)CO3 in iron red glazes enhances redness.

Soda
Hamer and Hamer (The Potter's Dictionary) note that soda along with minor iron quantities generates blue colors, while a higher iron concentration encourages red; details are scant.

Phosphorus
Bone ash is reported by many to improve the predictability of red hues; some opt for ferric phosphate instead of using red iron oxide along with bone ash (which contains substantial calcia).

Boron
Hamer and Hamer (The Potter's Dictionary) stated that boron mixed with small iron quantities produces blue, but specifics are lacking. Rhodes (Glazes for the Potter) mentions blue from iron and boron, but again, details are minimal. Obstler (Out of the Earth, Into the Fire) mentioned that boron intensifies green in iron celadons; specifics weren't provided. Hesselberth and Roy's Waterfall Brown (Mastering Cone 6 Glazes) gains green from iron, utilizing a formulation with 50% more boron than usual along with more soda. Genuine boron iron greens seem to necessitate very low calcia content, achievable only through frits since natural borates contain significant calcia.

Titania
Weyl (Coloured Glasses) observes that iron appears green when it serves as a network modifier, while titania reverts it to brown by functioning as a network former. A 4% titania presence in my experiments using calcium iron glaze affirmed this.

Migration of Iron
Two unpublished reports exist regarding subsurface iron migrating to the glaze surface. Ron Roy noted this in conditions of strong reduction, but not in oxidation under comparable conditions. Hank Murrow believes that fluoride in a glaze helps facilitate iron migration to the surface, supplanting a percent or less of cryolite in his glazes to attain this.

Sankey Iron Red
Custer feldspar 44g
silica 16.5g
bone ash 14g
red iron oxide 11g
EPK 10.5g
talc 10g
lithium carbonate 3g
Bentonite 2g
COE: 6.8x10-6/K
calcia: 10% molar
Stoneware (Tucker Smooth White)
Source: Kevin Baldwin, adapted to local clays
Painted on bisque, fired cone 6 electric, one-hour hold at 950°C. The vase stands at 7 cm high. A very even color appears, provided it exceeds 50 μm thickness, developing red crystals against a black background with visible depth. It tends toward black in its thinner areas. Among the iron reds I have tested, it is by far the most reliable, and I have selected it for my dinner set. Its expansion is high, but it aligns excellently with my clay, which has a relatively low expansion rate (6.64x10-6/K), likely due to its high potash content, which augments both elasticity and tensile strength.

Microphotos at approximately 60x magnification. X-ray analysis confirmed that all the crystals consist of pure iron oxide; Fe2O3 appears red, while FeO manifests as black. Red crystals were assessed at three different depths, demonstrating that a soak at 950°C mainly oxidizes the surface iron crystals to red Fe2O3 while the embedded iron remains black FeO, potentially as a result of oxygen movement to the surface due to chemical forces or surface oxidation. The glaze viscosity at 950°C is excessively high to support physical crystal sorting.

One hour hold at 950°C

No hold

Scanning electron microscope photo of Fe2O3 crystals Borate Iron Red:
Gerstley borate 32g
silica 30g
Custer feldspar 20g
red iron oxide 15g
talc 14g
EPK 5g
bone ash 6g
Bentonite 2g
COE: 5.6x10-6/K
calcia: 14% molar
Stoneware (Tucker Smooth White), thrown and trimmed.
Source: published numerous times under various names
Dipped on bisque, prefire thickness of 0.47 mm. Fired to cone 6 electric, with a two-hour rise to maximum temperature, held there for 10 minutes, and kiln off until soak temperature was reached (typically 30 minutes), held for the soak period, then kiln off (5 hours to reach 200°C).

I conducted a series of runs with the same bowl, varying only the soak temperatures. The first round involved a prolonged soak, allowing all potential crystals to develop. The microphotograph (about 50x) displays two distinct components in this glaze; one group creates very minute crystals or crusts at the surface, resulting in a rust color with extended soak periods. The second component without crystallization often appears yellow, occasionally vivid. It likely represents ferrosilite. Under specific temperature regimes, ferrosilite is colored a muted red by iron. Bright colors seem to correlate with surface components; the other microphotos shown are typical of the variety of colors observed. X-ray analyses indicate the surface crusts are thinner than 4 μm, rendering reliable analyses challenging due to the 20kV electrons’ effective penetration depth.

Black gradually permeated the bowl with multiple firings (the 980°C at right was the eighth in the series). Thus, if an expected color fails to manifest with this glaze, consider refirings, but executing too many may not yield desirable results. My most intriguing colors typically appeared with moderate-length soaks in the 900-980°C range.

X-ray analysis of the final glaze (the 980°C photo) exhibited considerable fine and medium-scale differentiation across the surface. Some regions had nearly pure silica (87%), whereas others boasted double the iron concentration compared to the total glaze formulation, with these parts being low in calcia. There was also evidence indicating the presence of calcium silicate. Matching the X-ray image to any visible features of the glaze remained unachievable.

This glaze behaved very well while mixed; it neither ran nor drooped excessively when applied thickly, as indicated by the lack of problems with the crisp horizontal edges of the 8 cm-diameter sugar bowl.

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5-hour hold at 870°C
 
30 minutes hold at 940°C
 
30 minutes hold at 980°C
 
glaze applied thickly on the outside, thinly on the inside, with a 1-hour hold at 920°C
 
scanning electron microscope photo of a 0.7 mm square of the surface, revealing typical crust-like differentiation. Calcium Iron:
Wollastonite 28g
EPK 28g
Fusion F2 frit 23g
silica 17g
red iron oxide 7g
nepheline syenite 4g
Bentonite 2g
cobalt carbonate 2g
COE: 5.5x10-6/K
calcia: 17% molar
Porcelain (Tucker 6-50), thrown and trimmed.
Dipped on bisque and fired to cone 6 electric. This started as an attempt at matte luster black but evolved into a microcrystalline glaze. A swift cool of this glaze produces a glossy black; a subsequent refire with a slow cool offers a mottled black and green semi-matte effect, and vice versa. Crystal size appears affected by the cooling rate, with cooling from 100°C at 50°C/hr to 800°C yielding approximately 2 mm diameter crystals. Faster cooling rates resulted in smaller crystal sizes, requiring 150°C/hr for a smooth surface, bearing a glossy look. If it is not fired glossy, iron concentrations exceeding 7% tend to precipitate out, forming a metallic layer between the green crystals. The bowl is about 10 cm in diameter.

The right side of the bowl was subjected to a 10-hour soak at 870°C post firing at 950°C. This generated a greenish-yellow crystal layer of needle-like formations radiating from a central point. The color may resemble either Hedenbergite or Ferrosilite, with crystal form aligning better with Hedenbergite. The crudeness of these crystals limits their X-ray output, mixing with background material. This also implies Hedenbergite presence.

Not an appealing or practical glaze.

I express my gratitude to John Stirling of Natural Resources Canada's Geological Survey for facilitating the scanning electron microscope-guided X-ray analyses.

Iron Glaze Chemistry

To explore the effects of calcia, magnesia, and phosphorus on iron within oxidizing-fired glazes, a range of mixtures was prepared. One base glaze contained none of the three oxides, while three alternative glazes mirrored base glaze composition, incorporating each oxide individually in significant amounts. The goal for each was a Seger ratio of 3.5 SiO2, 0.4 Al2O3, and 0.25 B2O3 (B2O3 and FeO were excluded from Seger ratios).

mix
recipe
molar Seger oxide base

28 silica
28 potassium carbonate
24 kaolin, EPK
10 iron oxide, red
10 frit, Fusion 367

0.602 
3.392 
SiO2
0.166 
0.935 
K2O
0.102 
0.575 
FeO
0.010 
0.059 
Na2O
0.072 
0.404 
Al2O3
0.045 
0.253 
B2O3

magnesia

30 silica
25 magnesium sulfate
25 kaolin, EPK
10 iron oxide, red
10 frit, Fusion 367

0.611 
3.497 
SiO2
0.163 
0.935 
MgO
0.098 
0.562 
FeO
0.072 
0.411 
Al2O3
0.043 
0.248 
B2O3
0.010 
0.058 
Na2O

calcia

32 silica
26 kaolin, EPK
22 calcium carbonate
10 iron oxide, red
10 frit, Fusion 367

0.615 
3.485 
SiO2
0.166 
0.939 
CaO
0.094 
0.533 
FeO
0.071 
0.405 
Al2O3
0.041 
0.234 
B2O3
0.010 
0.055 
Na2O

phosphorus

27 silica
26 potassium carbonate
23 kaolin, EPK
15 iron phosphate
9 frit, Fusion 367

0.587 
3.494 
SiO2
0.157 
0.936 
K2O
0.100 
0.596 
FeO
0.070 
0.418 
Al2O3
0.041 
0.246 
B2O3
0.033 
0.197 
P2O5
0.010 
0.057 
Na2O

All potassium and magnesium salts need to be dehydrated at 220°C before weighing due to their hygroscopic nature (particularly magnesium sulfate). Each glaze mixture was blended in a ball mill until uniformly fine. Although a 200 mesh sieve attempt was employed on the dry mixes, the hygroscopic clumping of soluble salts rendered it impractical requiring manual removal after mixing with oil.

Each tile glaze utilized four 1/4 tsp portions (1 tsp=5 ml) of dry materials, blended with 3/4 tsp corn oil since water was not an option due to the soluble salts needed for oxide separation. They were subjected to firing at 950°C (cone 10), cooled, then held for an hour at this temperature.

As optimal results were noted with maximum phosphorus from these mixtures and calcium was demonstrated as necessary, an additional mix was formulated using bone ash (a source of calcium) to yield larger phosphorus amounts. Ultimately, the composition yielding the most favorable coloration was trialed with 0-25% added red iron oxide. Some of the 54 test tiles are presented. Though not particularly ornamental, they reveal several functional results for oxidation firing above 950°C:

1. While excessive calcium is known to shift iron glazes to brown, a minimal calcium presence is necessary for producing red from iron.
2. To achieve an intense red, phosphorus is also essential; however, excessive phosphorus variations can result in a brown hue.
3. When magnesium is added to both, the red transitions toward orange, but too much magnesium can cause a grayish glaze.
4. The finest iron color outcomes from this series occurred with approximately 0.10 molar of CaO, 0.03-0.06 of MgO, and 0.01-0.02 of P2O5.
5. Using a base formulated with these proportions heated to cone 6, the best reds emerged with 9-14% added red iron oxide.
6. Inclusion of 1% cryolite (fluorine) may enhance surface brightness of the red slightly, but risks foaming and spitting issues during firing if used excessively or inadequately divided.

Only one phosphorus-iron mineral that is red has been located: Simferite (Li(Mg,Fe+++,Mn+++2(PO4), which remained absent in my test series. Park and Lee identified a magnesium-iron compound in their red samples; however, it seems they parsed the role of phosphorus as a means to sequester calcia so that it wouldn't disrupt magnesioferrite formation. This cannot be simplified; no red was achieved in any glaze lacking calcia, with merely a trace of very dark red in glazes containing calcia yet void of phosphorus.

My current working theory is that a calcium-phosphorus compound serves as a promoter—potentially a catalyst—for the oxidation of FeO to Fe2O3, or it may instead find ways to bind substances that inhibit oxidation, though this seems less plausible.

Notes:
Test were originally designed for cone 6. Boron, usually sufficient for this, was included. Even though the magnesium mix melted at cone 6, all mixes containing it became viscous and foamy upon melting. Cone 10, the maximum for my kiln, assisted with some mixes but proved inadequate for others.
Every stray cat from the neighborhood was attracted to my kiln's external vent due to the aroma from the corn oil!

Microphotos

Presented is the setup utilized for capturing microphotos. My microscope is a standard full-frame model - no condenser, fine focus, stage movement, or expensive extras. The objective lens is a 5x 0.1 NA. Low NA results in a broad depth of field, which is important for viewing solid objects. The eyepiece is a 10x periplan, carefully selected from various brands for clarity over the field optimal with this objective.

My camera is a basic Canon PowerShot, configured to focus and meter solely on a central square displayed on the LCD in P mode. This represents the least expensive camera yielding images sharp enough for web publishing. Many electronic brands offer poor resolution lenses or subpar autofocus systems. Its 3.2 megapixels suffice for web photography.

When using it, the camera is mounted on the eyepiece, shifted until the microscope's illuminated circle fully fills the screen, then zoomed to ensure the edge of the circular view aligns with the screen’s edges. This standardizes magnification, requiring only one measurement session. The microscope is focused until the LCD shows a sharp image, followed by shutter activation.

The total expenditure for the microscope plus camera is $340 in Canada. Both instruments are effective when utilized independently.

If the 50x magnification feels excessive, remember that cameras operate similarly to the human eye: if you can see it, you can photograph it! I find that placing magnifying glasses in front of the lens—particularly a 4x jeweler's loupe—proves very useful for capturing images of small insects.

Author's permission to republish

The materials showcased on my site, apart from those specifically credited to others, are protected by Copyright © John Sankey, -, under the Berne Convention, solely to uphold everyone's right to free use. Anyone can copy, link to, or circulate them as broadly as they wish, provided this copyright notice and permission for further copying accompany all reproductions. This is the only restriction I set; I expect them to remain completely free for all. No one is permitted to impose further restrictions on their use, be it through copyright claims, physical copy prevention, or any other methods. A reference link or credit is always appreciated. John Sankey

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