|Monthly Tech-Tip |
Kovar metal is thermal expansion compatible with borate glazes. You can actually glaze and fire it like pottery at cone 05, 06!
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Kovar metal has a thermal expansion to which a borosilicate glass can be matched. The match can be so good that bond integrity can be maintained over a wide range of working temperatures. The metal-to-glass bond is accomplished by a thin layer or cobalt or nickel oxide (this works well because Kovar is a nickel-cobalt alloy). And bond strength can be deduced simply by color! Pieces must be fired up and down quickly to avoid oxidation of the surfaces that do not have a glaze cover.
Amazingly, common hobby ceramic glazes for cone 05-06 can match the thermal expansion of the metal. A feasible approach is to simply get samples of the clear base glazes from a range of manufacturers and try them (many even have low, medium and high expansion versions of their base). These glazes have high binder contents and will paint onto any surface and stick well (although drying time will be lengthy).
A problem with the process is that gases are generated during decomposition of the binder in the glaze and these create bubbles in the molten glaze. However the glaze, which is just a suspension of fine mineral powder particles, requires a binder to make it stick. A solution is to have the oxide chemistry of the commercial glaze you are using analyzed (see link below) and then create your own glaze recipe to produce the same chemistry (this can be done in an account at insight-live). If you make your own then you can select only non-gasing materials (e.g. frits), choose a binder that gases early in the firing, choose a binder that does not gas (e.g. VeeGum) or find a way to apply and dry it without the need for a binder. An advantage of having your own recipe is that you can tune the thermal expansion, melting temperature, surface durability, etc by chemistry control (the resources to learn and do that are provided on this webiste).
The commercial cone 04 clear brushing glaze (on the left) works really well on our clay bodies so I sent it away to be analyzed (about $130). That revealed high Al2O3/SiO2 levels, this explains its resistance to crazing on our clay bodies and, even better, indicates high durability. In my account at insight-live.com I was able source the same chemistry from two Fusion frits (plus a little kaolin and silica). The melt fluidities are almost identical (my G3879 has a little more surface tension). I needed to make a dipping glaze version and chose a method that would produce a thixotropic slurry. One caution: An assay lab cannot analyze the complexities of a colored glaze, instead focus on the base clear and add stains to that. The first two-gallon bucket made saved the development cost plus more! And knowing the recipe made it possible to adjust for even lower thermal expansion. Another plus: I can now make my own low SG or high SG brushing version.
Kovar Metal at Wikipedia
Kovar compared to other metals
|By Tony Hansen|
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