|Monthly Tech-Tip |
Rutile is the mineral name for natural crystals of titanium dioxide. However in nature rutile is always contaminated by other minerals (especially iron but also things like tantalum, niobium, chromium and tin, the analysis provided here is obviously a simplification). In ceramics, the term 'rutile' is generally understood to refer to the brown powder into which these minerals are ground. Industry accepts up to 15% contaminants (below 85% titanium is called ilmenite). Rutile is considered an impure form of titanium whereas ilmenite is considered as FeTiO3. Grades of rutile are sometimes named after one of the impurities. Rutile is employed in many industries, ceramic uses are minor in comparison. There are large reserves of rutile in the world so supply shortages are related to other factors. Manufacturers often blend ores from different deposits, at times from different parts of the world. And they improve the TiO2 content by furnace-processing. Large users of rutile will often track batch numbers and test when the number changes. A situation can sometimes be dealt with by adjusting the amount of rutile in the recipe, firing differently or milling it. In more serious cases additions of iron or pure titanium might be needed.
Rutile is available in light tan calcined ceramic grade powder (light rutile), darker uncalcined powder (dark rutile), and granular form. Either grade of powder is normally ground very fine (e.g. 325 mesh). In glazes it is better to use a calcined grade (since the decomposition of raw rutile during firing could be a source of glaze imperfections like pinholing and bubbles). In our experience the LOI (weight loss on firing) when calcining rutile from our suppliers is less than 1%, so we are getting a calcined material.
Rutile produces many crystalline, speckling, streaking, and mottling effects in glazes during cooling in the kiln and has been used in all types of colored glazes to enhance the surface character. It is thus highly prized by potters, many attractive variegated glazes are made using it. Many potters would say that their living depends on their rutile supply!
Rutile is very refractory in oxidation, even a mix of 50% borax alumina-free frit like Ferro 3134 will not melt it in a crucible. In reduction, the improvement in melting will depend on the amount of iron present.
In ceramic glazes rutile is more often considered a variegator than a colorant. As little as 2% can impart significant effects in stoneware glazes. It is normally used in combination with a wide range of metal oxide and stain colorants to produce surfaces that are much more visually interesting. In glazes with high melt fluidity (e.g. having high boron), large amounts of rutile (e.g. 6-8%) can be quite stunning. The rutile encourages the development of micro-crystals (it is crystalline itself) and rivulets. Since rutile contains significant iron its use in combination with other colorants will often muddy the color that they would otherwise have or alter it if they are sensitive to the presence of iron. Even though rutile generally makes up less than 5% of stoneware glazes that employ it, they are often called 'rutile glazes' in recognition of its dramatic contribution.
Excessive rutile in a glaze can produce surface imperfections. In addition, when rutile is employed in higher percentages (e.g. 5%+) a given percentage might work well whereas a slightly higher amount can look drastically different. Such situations are vulnerable to chemistry changes in the supply of rutile. Thus people will often do a line blend trying a range of percentages to determine an optimal amount.
In glazes rutile can be quite sensitive to the presence of opacifiers. While an unopacified glaze glaze might appear quite stunning, the addition of a zircon opacifier will usually drastically alter its appearance and interest because the variegation imparted is dependent on the glaze having depth and transparency or translucency. Strangely rutile and tin, another opacifier, can produce some very interesting reactions and it is quite common to see tin in amounts of up to 4% in rutile glazes. In these cases the tin appears to react in the crystal formation rather than opacify the glaze.
Pure rutile powder, although its color makes it appear to be a crude ground mineral, normally contains 95%+ titanium dioxide. However this does not mean that you can use a 95% titanium:5% iron mix and get the same result in a ceramic glaze (obviously line blending would be needed to match the amount of iron). The mineralogy and significant other impurities in rutile are a major factor in the way it acts in glazes (not easily duplicated using a blend of other things). Sometimes the special effects that rutile produces in glazes are also partly a product of a coarser grade (larger particle size). These likewise cannot be easily duplicated by more refined materials. Unfortunately the trend at some mining operations (at least in Australia) is to fine grind the rutile on-site, making it more difficult for ceramic operations to obtain the coarser grades.
Although rutile will normally stain a glaze brown or yellow, its crystallization effects can significantly lighten the color of iron glazes. Higher amounts of rutile in stoneware glazes will often contribute glaze imperfections.
Granular rutile is sometimes used in bodies and glazes to impart fired speckle.
Rutile is used for special effects in leaded glazes and can form up to 15% of the recipe.
Rutile can be used as a tone modifier to soften the more potent colorants.
These two cone 6 mugs have the same glaze recipe: GA6A Alberta Slip base. 4% rutile has been added to each. They were fired in the same kiln using a slow cool schedule. The recipes and chemistry are shown below (the latter gives a clue as to why there is no blue on the right). The mug on the left is the traditional recipe, 80:20 Alberta Slip:Ferro Frit 3134. Frit 3134 melts at a very low temperature and a key reason for that is its near-zero Al2O3 content. Al2O3 in glazes stiffens the melt and imparts durability to the fired glass (normally we want adequate levels in functional glazes). When Al2O3 levels are low and cooling is slower molecules in the stiffening glass have much more freedom to move and orient themselves in the preferred way: crystalline (fast cooling produces a glass). Thus the rutile in the glaze on the left has had its way, dancing as the kiln cooled, producing all sorts of interesting variegated visual effects. The glaze on the right employs Ferro Frit 3195. It has lots of Al2O3 and has contributed enough to stop the rutile dead.
2, 3, 4, 5% rutile added to an 80:20 mix of Alberta Slip:Frit 3134 at cone 6. This variegating mechanism of rutile is well-known among potters. Rutile can be added to many glazes to variegate existing color and opacification. If more rutile is added the surface turns an ugly yellow in a mass of titanium crystals.
The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. However on the outside the dried glaze was over-sprayed with a very thin layer of titanium. The dramatic effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide. It is a more consistent source of TiO2 than the often-troublesome rutile.
The rutile blue variegation effect is fragile. It needs the right melt fluidity, the right chemistry and the right cooling (during firing). This is Alberta Slip GA6C recipe on the right (normal), the glaze melt flows well due to a 20% addition of Ferro Frit 3134 (a very low melting glass). On the left Boraq has been used as the flux (it is a calcium borate and also melts low, but not as low as the frit). It also contains significant MgO. These two factors have destroyed the rutile blue effect!
This is Alberta Slip (GA6C) on the left. Added frit is melting the Alberta Slip clay to it flows well at cone 6 and added rutile is creating the blue variegated effect (in the absence of expensive cobalt). However GA6D (right) is the same glaze with added Tin Oxide. The tin completely immobilizes the rutile blue effect, it brings out the color of the iron (from the rutile and the body).
This is a common problem with these glazes. The visual effect is very compelling but also punishing! Potters experiment with higher bisque firing and soaking during bisque. They try cleaner clay bodies. They employ long hold periods at temperature in the glaze firing. But the problem persists. The solution is actually simpler. These glazes have a high melt fluidity and enough surface tension to hold a bubble static during soaks at temperature (no matter how long you hold it). It is better to cool the kiln somewhat (perhaps 100F) and soak at that temperature. Why? Because the increasing viscosity of the melt overcomes the surface tension that maintains the bubbles. You may need to cool more or less than 100 degrees, but start with that.
Originally popularized by James Chappell in the book The Potter's Complete Book of Clay and Glazes. It is loved and hated. Why? The high Gerstley Borate content makes it finicky. But the magic ingredient is not the GB, it is the rutile, Rutile makes the cobalt and iron dance. This recipe actually produces a number of different mechanisms of variegation. Color and opacity vary with thickness. Small rivulets of more fluid glass flow around more viscous phases producing micro-areas of differing colors and opacities. Titanium crystals sparkle and calcium-borate creates opalescence. Bubbles of escaping gases (from GB) have created pooling. Small black speckles from unground or agglomerated particles of iron are also present. Surprise! This is actually Ravenscrag Floating blue. All the visuals, none of the headaches.
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
Left: GA6-C rutile blue glaze on a brown stoneware. The 4% ceramic rutile powder gives the blue variegated effect. Right: We ball-milled our granular rutile and then screened it down to 325 mesh and put that into the same glaze. The results are the same. So if any of your rutile glazes ever lose this effect with a new supply of the material the cause could be that it has not been milled sufficiently fine. Finer rutile powders are browner in color.
The glaze is G191T (a variation of G1916Q). Firing was cone 04 drop-and-hold with slow cool. Sometimes a raw colorant is advisable over a ceramic stain. At low temperatures stains are almost universal. But in this case, the orangey-yellow color that rutile produces merits further testing. On the red body (Plainsman L215) the color is barely perceptible, but on the light Buffstone body it is working well. The variations in thickness highlight contours better than what a stain would do.
A closeup of a cone 10R rutile blue (it is highlighted in the video: A Broken Glaze Meets Insight-Live and a Magic Material). Beautiful glazes like this, especially rutile blues, often have serious issues (like blistering, crazing), but they can be fixed.
Rutile variegates glaze surfaces. But it also opacifies at higher percentages. The blue effect is a product of crystallization that occurs during cooling, it is thus dependent on a slower cooling cycle, especially above 1400F. This is GA6-C Alberta Slip glaze with 4, 5 and 6% rutile. At 6% the rutile crystallization has advanced to the point of completely opacifying the glaze. At 5% the blue is still strong, even on a buff burning body. The loss of color occurs suddenly, somewhere between 5 and 6 percent. Rutile chemistry varies from batch to batch. The blue develops differently on different bodies. So do you want to play "at the edge", with 5% in the glaze, in view of these other factors and the finicky firing curve needed. Change in any of which could push it into the blueless zone?
These glazes are both 80% Alberta Slip, but the one on the right employs 20% Ferro Frit 3249 accelerate the melting (whereas the left one has 20% Frit 3134). Even though Frit 3249 is higher in boron and should melt better, its high MgO stiffens the glaze melt denying the mobility needed for the crystal growth.
Ceramic glaze variegation refers to its visual character. This is an overview of the various mechanisms to make glazes dance with color, crystals, highlights, speckles, rivulets, etc.
In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.
Report on the composition of Nigerian Rutile
Rutile on Wikipedia
Consolidated Rutile Ltd in Australia
Rutile information website
Rutile at WebMineral.com
Titanium Dioxide Toxicology
|Oxides||TiO2 - Titanium Dioxide, Titania|
Metallic based materials that impart fired color to glazes and bodies.
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
|Frit Softening Point||1600C M|
|Density (Specific Gravity)||4.20|
|Body Color||Rutile sand (granular) can be used to add speck to bodies.|
|Body Color||Rutile can be added to low fire bodies to make them burn golden yellow.|
|Glaze Color||Rutile produces many shades from pale straw to tan to cinammon brown to orange brown. Alkaline glazes experience less of the classic rutile crystalline effect. Color intensifies with increased amounts and tends to be darker in reduction.|
|Glaze Color||5-8% added rutile can give powder to deep blue colors in reduction. These colors can be brilliant and mottled with shades of browns and tans. Rutile effects can be hard to Like copper reds, rutile blue reduction effects depend on adequate silica being available (7 times or more that of alumina). Classic rutile blue glazes are best made by using rutile in fluid glossy bases. The amount of iron in the rutile (or from the body) determines the degree of blue since its reduction to FeO is a key to the effect. Since rutile is variable in makeup, changes in the iron content will change the blue color. The fact that titanium dioxide additions do not make good blues suggests that the iron-titanium mix is the key to good color. Silky matte variegated blue glazes can also be made with rutile additions. If you employ this type of glaze be sure that you have a large supply of rutile on hand and test new supplies throughly.|
|Glaze Color||In zinc glazes powdered rutile develops an orange-tan color.|
|Glaze Color||Rutile can produce soft tan colors due to its iron content, especially in matte glazes.|
|Glaze Color||Rutile and cobalt can crystallize to form green glazes.|
|Glaze Matteness||Even in small amounts, rutile tends to matte the surface of leaded glazes.|
|Glaze Opacifier||As an opacifier rutile is economical compared with pure titanium and it is employed where white shades are not required. It is also used to stain pottery bodies and glazes (yellowish, orange and tan colors are most common because of its iron content).|
|Glaze Variegation||Rutile has the unique property of breaking up and variegating the color and texture of glazes, it is quite popular in tile and art pottery for this reason. However this effect depends on the rutile powder being coarse enough to act as an adjunct (the finer grades disperse more readily into the glass matrix). The addition of 4-8% rutile to many stoneware glazes can make an otherwise drab or flat glaze become much more interesting. When used in combination with colorants, it can be very effective at improving the character, however it will affect the color. When used with added tin streaking and mottling effects can be enhanced, especially in lead glazes.|