Matt And Glossy Glazes From "mastering Cone 6"

[lwa]

Hello All,

 

I'm new here, and something of a beginner to pottery. I recently bought a old small electric kiln --- basic thing controlled by two dials: power 0% to 100% and target temperature up to 1300*C, and a flick switch for 'hold' or 'power off' once above the target temperature --- and am trying to work out some basic glazes.

 

I've just fired two glazes based on Glossy Base 1 and High Calcium Semi-Matt 2 from Hesselberth and Roy, and am baffled by the results. The glaze based on High Calcium Semi-matt has come out rather glossy, and the glaze based Glossy Base 1 is matt --- very very matt.

 

Can anyone shed any light on what might be going on?

 

 

I fired up to Cone 6 (cone bent over until the tip is touching the base) over 9 hours. The last 100*C took an hour and half, the cone was over at a little over 1200*C. (I can roughly check the temperature according to the pyrometer by turning the target temperature dial and seeing when the relay kicks on/off.)

 

Fired down as follows:

- Natural drop to 1000*C (1830*F) took around 35 minutes

- Hold 1000*C (1830*F) for 30 minutes

- Hold above 930*C (1700*F) for 30 minutes

- Hold above 900*C (1650*F) for 1 hour

- hold above 870*C (1600*F) for 1 hour

- hold above 840*C (1545*F) for one hour

- kiln off

 

The only thought I have is that Glossy Base 1 has MgO 0.279 in the unity formula where as High Calcium Semi-matte has only an incidental amount (0.04) and maybe something in the firing cycle is causing the magnesium to give matt but not the calcium in the high calcium semi-matte. The overall rate of cooling is not so different to that recommended in H&R, but obviously they assume a continuous ramp down, but because of how my kiln works I've used a series of holds -- could that cause some effect?

 

Any thoughts gratefully received!

 

Liam

 

 

[clay lover]

I use these a lot, with a similar cool down.  I'm guessing the glossy was way too thin.  The matt being  glossy, , that one for me is glossy where thick, matt and lighter where thinner.  Just my thoughts.  I'm no glaze expert.

[lwa]

Thanks for the thoughts. If I understand correctly, you're saying the one that turned out matte but should be glossy I should try thicker, and the one that turned out glossy but should have been matte maybe try thinner

 

 

For each glaze I did do a few test bowls with a variety of thicknesses and all had the same surface qualities.... Though maybe none with the glossy base 1 were thick enough. I'll run the kiln again on Friday and try some more systematic tests regarding thickness,

 

Thanks, Liam

[clay lover]

Did you use the schedule that is in their book? Read the whole book, there is much info in there that will help you.  Not just the glazes, but how to use them and fire them.

 I'm thinking you might have gotten to peak temp a little fast, the matt glazes need some soak at top temp to be matt.  I use a 30 minute soak at 2195* F  to get a solid ^6 in my 7 cubic foot kiln.  The longer you soak the lower the temp.  It's still a ^6, but the time involved is about glaze development.

I use a similar down fire, but my kiln runs 16 hours with the slow ramp down.  9 hours seems awfully fast to get good glazes.  Is that from kiln on to kiln off, ramp up and down?

[angela_w]

I'm sorry I can't offer any technical advice (as I'm also a glaze newbie), but I will say that I just tested Glossy Base 1, also with a slow cooling, and found the result to be more of a smooth, satiny finish—not matte, but definitely not a super shiny gloss. In my testing, I found both "Glossy Clear Liner" and "Glossy Base 2" to be true gloss finishes. I'd definitely recommend trying Glossy Base 2 if that's what you're looking for.

[clay lover]

Remember that the colorants can flux the glaze more, so a blue or green with the oxides added will be shinier than the base without colorants.  sorry , should have added that sooner.

[lwa]

Thanks folks for the additional thoughts.

 

clay lover, sorry - should have been more clear - it was 9 hours from kiln on to cone 6 down... Actually checking my notes more carefully it was 9 hrs 30 - then the cool down as described above - an additional 4 hrs 30.

 

I wasn't exactly following H&R's schedule, partly because with my manual kiln it seemed easier to make a series of holds rather than smooth ramps on the way down. Based on your suggestion, I think on the way up I might set a top temperature of 2195*F (1200*C) then hold to get cone 6 down rather than letting the temp keep rising as I did last time.

 

On the way down, H&R do say in the text that a fast drop from top temperature for the first 100-150*C is fine. Though, I've just noticed that on their suggested program, on ramp number 4, there is an inconsistency between the *C numbers (which I used) and the *F numbers. It says to slow the cooling from 1000*C or 1900*F, but 1900*F is 1040*C - maybe that 40*C window is important for calcium crystals?

 

For oxides I added 1% Copper oxide, 2% red iron oxide to the high calcium semi-matte...

 

angela, thanks - I might try to mix up some glossy base 2 before firing again on Friday

 

Cheers all, Liam