Monday, 28 March 2016

So where did this glaze go wrong?


Ahem .. well.. adding an extra layer of the matt gold glaze (a simple recipe containing kaolin, manganese dioxide and copper oxide) and refiring them a cone higher didn't work terribly well! The firing yesterday went to cone 8 (and 9 as well accidentally) .. yet the gold colour of the manganese has still failed to emerge,  or rather it has appeared but only sporadically. Most of the glaze has come out dull and way too dark (as per the first attempt) and I'm rather baffled as to what has gone wrong.

I tested the same glaze on small head sculptures some years ago and they came out pretty well .. admittedly the finish wasn't perfectly even:

Ceramic sketches, 2008

And decided to try this again now as I figured this glaze would work well on some of the more textured pieces I've been making lately. So the finish above was what I was aiming for, but this was a new batch of glaze, a new supply of manganese and copper oxide and these were some of the results which were very different, and on the whole, didn't live up to expectations:

Tea caddy, carved from a solid block of clay
Bottle vase, height 3.5 ins


Vases

Another larger vase (not shown) barely had any gold hue at all, and two layers were brushed on, so I would be surprised if glaze thickness is the issue here. Having said that I did use a small amount of CMC suspender to avoid brushmarks and perhaps this reduced the glaze thickness? It is puzzling and perhaps the new batches of materials have something to do with it. Also my electric kiln elements are new so the firing is faster than normal .. could it be insufficent heat work in parts of the kiln? Certainly the cones dropped very quickly at the end, and strangely, cone nine, which was placed directly behind cone eight, bent exactly in tandem, something I've never seen happen before. I assume this was because it's slightly further away from the spy hole, but would a centimeter or two make so much difference?   

Although the colours are muted, I feel the matt metallic finish is very effective on this heavily textured vase:

Oval vase, height approx. 5 ins

I guess dipping could improve application, but I don't fancy the health hazards of making a bucket of glaze which contains 30% metal oxides! The cost for a start would be fairly prohibitive.

Well, if anyone has worked with this kind of glaze and would like to share ideas, please do leave a comment.

On the bright side, this dotty bowl was glazed with a new batch of my translucent dolomite recipe, and I was very happy with the result:

Bowl, height approx 3.5 inches

And this little hanging vase with nezumi style shino came out okay too .. a piece I've been meaning to fire for ages:


Anyway, it's Bank Holiday here today so the weather is grim, grim, grim .. high winds and lashings of rain! Which partly explains why I'm indoors tapping away on the PC. Have a great week everyone, and thanks for reading!

Update: I now think there's a good chance that the dullness and lack of gold finish is due to the china clay content being too high, combined with the clay body of the pots being very refractory. This could explain why the glaze hasn't melted sufficiently ... so, the next step is too make another batch with a higher proportion of metal oxide, and fire it a third time. However,  next time I will try it out on just a couple of pieces! Like many potters I suspect, I am sometimes guilty of ignoring the old saying "Test, test and test again!"



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                                             Never give up, the best is yet to come










 









Sunday, 27 March 2016

The quest for a matt grey glaze continues..


So, it's been a while and I'm still trying to develop that elusive, matt "grey" glaze! So far I've concluded that black iron oxide only produces grey either in an engobe which isn't fully vitrified, or in larger percentages which gives a very dark grey-brown, verging on black. Ideally, I'm looking for a light grey like this, but with more sheen to it:


Test tiles, fired in oxidation to cone 9

The two test fires on the left were pleasing .. made with an engobe (clay, potash feldspar and a little raw ash), plus small amounts of black iron oxide, nickel oxide and cobalt oxide. But since I started using a new batch of black iron I've been unable to repeat it, instead just obtaining various shades of green-grey or brown. Incidentally, the test piece on the left was made by mixing red earthenware with a coarse raku body .. I've found that this combination can withstand cone 8/9 firings in an electric kiln).

The following tiles were made with a matt ash glaze (adapted from John Jelf's ochre ash glaze recipe) and varying amounts of black iron, cobalt and manganese dioxide (instead of the nickel):

Test tiles fired in oxidation to cone 9
The test second from left (above) was close but no cigar .. and funnily enough this was made with a cheap black iron bought from Ebay which looked more impure and browner in its raw state than those bought from pottery suppliers. The other sources of black iron produced much browner hues:



The tile on the left looks grey but is really an extremely dark brown (over 4% black iron oxide).

The most convincing grey I've concocted so far is with another engobe recipe, here applied thinly by dipping a bisqued pot, then fired to cone 7:


Still too dark to be used on its own, and possibly verging on black rather than grey. The engobe recipe if anyone would like to try it:

HVAR ball clay    50
China clay          50
Potash feldspar   20

plus, as a percentage of the total dry weight: 8% black iron oxide, 3% manganese dioxide, 1% cobalt oxide
                                         
Since then, I've come across a couple more recipes which use a combination of rutile, cobalt and nickel to make grey, so this provides yet another avenue of research. Ofcourse one of the problems being that rutile can vary alot in terms of its chemical make up .. 

Anyway,  I've still found time to produce a few pots here and there. Below are some pieces fired fairly recently in  the electric kiln. The dolomite glaze on the first three pieces is a mixture of the dolomite glaze I normally use ( many thanks to Rachel Wood for this recipe, found in Techniques Using Slips, by John Mathieson ), with a transparent ash glaze. The result is less opaque and allows more of the iron oxide brushwork to show through:




 Plus two pots with the old dolomite glaze:



And finally, a couple from the latest firing, both with metallic glazes:

Lidded container. Metallic over matt ash glaze

Sculpture, height approx 8 ins

Most of the pieces in this firing were coated with a matt gold glaze (active ingredients: manganese dioxide and copper oxide) applied by brush and came out rather under fired. I was probably over-cautious in firing them to cone 6/7 only, as I didn't want the manganese to flux and become too shiny. Also, the gold colour was rather patchy, so I brushed on another layer and am re-firing them today, to cone 8. It will be interesting to see what effect the extra temperature has on the glaze, and hopefully I may have a few good pieces to show next week!

Happy Easter! And thanks for reading.

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Never give up, the best is yet to come!