|Monthly Tech-Tip |
Alternate Names: Stannic Oxide, Tin(IV) Oxide, Tin Dioxide, SnO2
Tin oxide is a white or off-white powder produced by oxidizing molten high grade tin metal. It is typically quite pure, some manufacturers have grades up to 99.999% purity. Tin oxide power is very fine-grained and it agglomerates, so glazes containing it need to be sieved to break down the small lumps (even a high speed propeller mixer often won't do it).
Tin oxide has long been used to opacify glazes in oxidation (make transparents opaque) at all temperatures. Hand decorated tin glazed earthenware of the 1700/1800s is the most famous use of tin in glazes (delftware-England, faience-France, maiolica-Italy). While many potters are keeping this tradition alive today most now use zircon based opacifiers instead. Thus any discussion about the use of tin oxide as an opacifier ends up comparing it with zircon products:
-Twice as much zircon is required to produce the same level of opacity.
-Like zircon, tin melts at very high temperatures and thus does not go into solution in typical glaze melts.
-Zircon will stiffen the glaze melt more than tin.
-Zircon will produces a harder glaze surface, but that surface can have cutlery marking issues.
-Zircon will reduce the thermal expansion of the glaze more than tin.
-The quality of the white color is different (tin tends to be more of a blue white, zircon a yellowish white).
-Tin is very expensive. That being said, it has double the potency and tin glazes can be applied thinner, these two factors can even out the cost.
-Zircon tends to have less of an effect on the development of metal oxide colors (e.g. tin reacts with chrome to make pink).
-Tin can often produce and maintain better gloss.
-While there are other products that produce varying degrees of opacity, none are as neutral and non-reactive as tin and zircon. Other opacifiers also tend to variegate the glaze.
-Tin does not normally opacify in reduction firings.
Tin is also a player in the development of ceramic colors, for example chrome tin pinks and maroons. Tin with iron in oxidation makes a warmer shade of brown than zirconium does.
Tin oxide is also a variegator. For example, tin can react with titanium and rutile to to completely transform the color and character of a glaze. Although tin is expensive, very little is required to produce stunning effects in many colored glazes.
Some claim that a little tin (not enough to opacify) will add extra smoothness and shine to many glazes.
Made in Italy by Oximet Srl.
This is a calcium matte base (as opposed to the magnesia matte G2934). The clay is Plainsman M390. 5% Zircopax was added on the left (normally 10% or more is needed to get full opacity, the partially opaque effect highlight contours well). 5% tin oxide was added to the one on the right (tin is a more effective, albeit expensive opacifier in oxidation). The PLC6DS firing schedule was used.
This is a cone 04 glaze on a terra cotta body. Two 300-gram test batches were made. Both have 5% tin oxide added. The one on the left was high-speed propeller-mixed for 10 seconds on a closed container. That was not enough, small agglomerates appear as white specks floating in the glass. The one on the right was mixed for 60 seconds. Now the tin particles, which are incredibly small, have been dispersed and can do their job of opacifying the glaze. Notice that 5% is not quite enough, more is needed.
This is a melt fluidity test comparing two different tin oxides in a cone 6 transparent glaze (Perkins Clear 2). The length, character and color of the flow provide an excellent indication of how similar they are.
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).
Same body, same glaze. Left is cone 10 oxidation, right is cone 10 reduction. What a difference! This is a Ravenscrag-Slip-based recipe on a high-fire iron stoneware. In reduction, the iron oxide in the body and glaze darkens (especially the body) and melts much more. The behavior of the tin oxide opacifier is also much different (having very little opacifying effect in reduction).
Right: Ravenscrag GR6-A transparent base glaze. Left: It has been opacified (turned opaque) by adding 10% Zircopax. This opacification mechanism can be transplanted into almost any transparent glaze. It can also be employed in colored transparents, it will convert their coloration to a pastel shade, lightening it. Zircon works well in oxidation and reduction. Tin oxide is another opacifier, it is much more expensive and only works in oxidation firing.
Both of these mugs were soaked 15 minutes at cone 6 (2200F), then cooled at 100F per hour to 2100F and soaked for 30 minutes and then cooled at 200F/hour to 1500F. This firing schedule was done to eliminate glaze defects like pinholes and blisters. Normally the GA6-A glaze crystallizes (devitrifies) heavily with this type of firing, but an addition of 1% tin oxide to the one on the left has prevented this behavior.
Tin is super expensive, so how much should you use in a clear glaze to get a white? It is a trade-off of cost and whiteness. Nothing else can make a glaze this white and opaque and these low percentages. Consider this: A tin-opacified glaze may only need to be half as thick as a zircon opacified one. Tin has other advantages over Zircopax. First, the percentage required could be half or one third. That takes us down to four or six times less tin being needed (Zircopax is five times less expensive at time of writing). These two factors mean thermal expansion mis-fits between body and glaze start off four or six times less likely to produce shivering or crazing! And tin affects the melt fluidity and thermal expansion half as much. On these samples, the higher percentage of tin seems to produce an even better glossy surface. Crawling is a classic issue with high-zircon glazes (because it impedes melt fluidity, that is what holds super thickly applied majolica glazes on the ware). Tin is the opposite; even though this recipe is high in strontium, and thus has a high surface tension, there is no indication of crawling with the tin addition. A final issue is cutlery marking, a common problem with zircon-opacified glazes. But not with tin oxide.
The body is Plainsman M390. Firing is the cone 6 PLC6DS schedule. Each horizontal row is a commonly-used base glaze. The top one is an MgO matte, the next one down is a calcium matte, row 3 is G2926B glossy and row 4 is Ravenscrag Slip+frit. The two mattes behave very differently from each other with the additions of opacifier. Thickly applying an opacified glaze will obviously affect visual character (column 4). Tin oxide fires whiter than zircon (e.g. Zircopax). If you like the G2934 recipe, consider the G2934Y variant for better melting.
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.
Metal oxide powders are used in ceramics to produce color. But a life time is not enough to study the complexities of their use and potential in glazes, engobes, bodies and enamels.
Glaze opacity refers to the degree to which it is opaque. There is more than meets to eye to the subject of opacity control.
|Oxides||SnO2 - Tin Oxide, Stannic Oxide|
Tin Oxide at Wikipedia
The Mineral Cassiterite
Opacifiers are powders that turn transparent glazes opaque by various chemical and physical mechanisms (and combinations of mechanisms).
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.
|Glaze Opacifier||As little as 4-7% can produce brilliant white, although it is more typical to use 8-10% for full opacity. However, be aware that even tiny amounts of chrome in the kiln will volatilize and combine with the tin to produce pink shades.|
|Glaze Variegation||Tin reacts with iron in fluid glazes to produce variegated surfaces. A good example is the Albany Slip 85, Tin 4, Lithium 11 glaze for cone 6.|