|Monthly Tech-Tip |
Yes, this is real, ordinary cow's milk can be used as a glaze on pottery! As explained below, this works in the domain of low temperatures, typically cone 04 on terra cotta (cone 06 is also possible but ware is not as strong).
Milk is not a glaze in the normal sense. Contrary to what is stated elsewhere, we find that it does not seal the surface, so water will absorb into porous clay fired at low temperatures (and the milk won't bond well with vitreous ware fired at higher temperatures). It is almost always most practical to use a terra cotta clay for this process, giving milk glazed ware all the benefits and limitations of any other terra cotta. It is thus not practical to put this type of ware in the dishwasher. The absorption issue can be alleviated by dipping ware in a silicone sealant to cover milked areas.
Although stated otherwise elsewhere, it is only logical and practical to glaze food surfaces, limiting the milk to the outsides of pieces (as decorative or a talking point). This is obviously more sanitary. Ware should be washed by hand with some effort to minimize waterlogging it. Extra attention must be given to using a glaze whose thermal expansion is compatible with the body (it is not under tension and in danger of crazing and not under excessive compression in danger of shivering). Since the glaze must be under at least some compression to avoid crazing it is important that it not be applied too thickly, this minimizes the danger of shivering and glaze compression issues. The G1916Q recipe works well for this since it is thermal expansion adjustable.
Firing. We have found that 610F works well (although we are doing tests at other temperatures to see how high it can go before burning off). Of course, pieces have already been bisque fired and glaze fired, so this is the third firing. I use no special schedule, just straight up at 350F/hr. That being said, since ware is already glazed inside, too rapid a rise in temperature has the potential of cracking pieces getting heated unevenly.
In the youtube video (link below) a Karelian potter uses this technique to make cookware. He immerses pieces, while still hot from the bisque kiln, intp a bucket of milk. After a few seconds he withdraws them and they steam-dry quickly. I did not preheat these Plainsman L210 pieces, they were just bisque fired to 1850F. The Russian potter claims to milk-fire his ware to 300-350 degrees, assuming that is celsius I fired these at 500F/hr to 612F (350C), then held for 10 minutes. The surface is smooth and pleasant-to-touch, it is odor-free. The potter claims it retains this surface over many years despite repeated oven use. This clay body, L210, is well suited since it fires to a smooth unglazed surface. Indigenous cultures throughout history have learned how to prepare, cook and store food in terra cotta clays like this, they withstand thermal shock better than vitrified stonewares and porcelains. And continue to service even if they do crack.
Left: Plainsman L215 terra cotta bisque and glaze fired at cone 04 using the G1916Q recipe. Right: L215 bisque fired to cone 04 and then glaze fired to cone 2 using the G1916V recipe. While glazing ware only on the inside can create glaze compression issues, that has not appeared to be an issue so far. I preheated both of the glazed mugs to 300F and quickly dipped each in milk (both steam-dried rapidly). I was careful not to touch the surface of either while they dried (since touch marks show up after fired). However, the one on the right did not work. While the milk did apply thickly enough to fire on normally, it has only created a film on the surface. After a couple of weeks in service, with repeated cold-to-hot and wash cycles the milk is shedding off! It thus appears the milk needs an absorbant body to form a bond, low fired terra cotta, like the mug on the left, appears to supply that best. Another issue is achieving a drip and bubble-free application, application using a milk-soaked sponge creates a variegated pattern, under more control.
The milk was applied to inside-glazed ware that I had preheated to 250F. This has not gone on as thick as usual so it appears it might be best to dip the pieces into milk and then pat them with a milk-damp sponge to break all the bubbles, remove the drips and even out the coverage.
The firing temperature: Both are fired at cone 04. Left: Zero4 porcelain, this fritware is totally dense and durable yet fired ten cones lower than typical. You can make it. As an appreciator of the process more than the product, just drinking out of this amazes me! Right: Milk glazed earthenware. The inside glaze, G1916Q, was fired onto L215 at cone 04. The 250F preheated mug was quickly dipped in milk after which it steam-dried. I then refired it at 610F to get this effect. To improve durability I dipped the outside in silicone sealant. The surface looks and feels like leather, drinking coffee out of this is an amazing experience!
Karelian potter produces glossy black pottery using milk as a glaze
|By Tony Hansen|
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