Saturday, June 30, 2012

Fixin' the Kiln

I've been working on my kiln the past few weeks. It hasn't been working quite right for me since I bought it a year and a half ago. I think it's been running too hot. Metallic lusters keep evaporating off my beads after only an hour or two in the kiln. And since almost all of my beads have some metallic luster, I need to get this problem fixed!

It's a Glass Hive Regular Guy kiln, and Mike and Pam at Glass Hive have gone above and beyond in trying to help me solve this problem. One issue I had was my kiln element was hanging down out of the ceiling because a fire brick had broken. They sent me an element replacement kit, with an element and new fire bricks, and I actually changed it out myself!

happy kiln :)
Coffee + tools + girl power + instructions I didn't read right because I'm blonde + a cheer-up from my kiln = replacement element installed pretty much okay I think!
Though the element did need to be replaced, that didn't solve my reduction problems. The next thing Mike and Pam suggested was that I drill a new hole for my thermocouple, about an inch above where it was sitting now. That would put it closer to the elements and they said it would give a more accurate temperature reading. I did that last night (with my girl power tool wielding confidence) and ran another test of metallic glass beads.

With the thermocouple adjustment, the silver glass reduction stayed for an hour longer than it did before, but still - it vanished - and that's no good for me.

The next thing I'm going to do is lower my kiln temps down even further, plus shorten my annealing time which was set too long for my tiny beads anyway, and I'm going to test metallic glass again tonight. I also might buy a stand-alone pyrometer so I can get an independent temperature reading. I've tried the activated charcoal trick, which had no effect on my beads (probably because my kiln was simply too hot), but I might try that again too.

I have a couple of customers waiting to see if I can do metallic bead special orders for them. They've been really patient since I have no date yet when my kiln will start behaving! Who knows, maybe tonight will be the night, when I find that perfect temperature and annealing time that works for my silver glass beads!

Have a great weekend! :)



6 comments:

Lori Bergmann said...

Crossing my fingers and toes that you get your kiln problems fixed soon! And love that happy face on the side of it! LOL

Glaudius said...

Metallic luster means reduction which means low or no oxygen. Could it be that you have too much oxygen which oxidizes the metal turning it transparent ?

If the bead is too hot when you retrieve it from the kiln it also can oxidize from air contact fast enough you will not see it.

I don't know your process so these are just a guess.

Karolen said...

Thanks for your comments Lori and Glaudius!
It's acting like it's an oxidizing problem. It's been driving me mad trying to figure it out. The Glass Hive thinks it might be iron contamination in the kiln.

FlyingWelshMan said...

Hi, just came across this blog and read through the thread of references to the kiln issues.

The date of the blog was quite a while ago so in all probability you've hopefully successfully sorted through all of your issues regarding metallic finishes etc.

If there still remain any issues I hope my following comments may be of some use, if they are not, please disregard the following.

The issues of reduction as described on the net are a bit varied in terms of interpretation when it comes to kilns and firing. A reduction environment is one where the oxygen is either eliminated or reduced. Strictly speaking an electric kiln does not do either of these things as the heat is not generated through incendiary methods. Normal dry air has an average oxygen content of around 20.9%. If the oxygen is not reacted by some form of exothermic reaction where the oxygen is used as an accelerant then this percentage doesn't change much with temperature rise. This remains true even for the extremely hi temperatures present in the kiln during firing and annealing cycles.

Obviously in a kiln that is heated by combusting both fuel and oxygen (as happens with the torch) then the total oxygen percentage present can be controlled and varied for a given firing cycle. Again for the purposes of using an electrically heated kiln this is not the case.

What will happen at different temperatures is that the oxygen present in the kiln’s atmosphere will have the chance to react with the metal’s present in the glass at the time of firing. These reactions can occur at very specific temperature ranges which could be as small as just a few degree’s or ten’s of degrees (the scale does no matter as long as the measurements are all given and recorded in a specific scale like Kelvin, Celsius or Fahrenheit). The second variable affecting reaction is time. The time at which the material is at a given temperature in order for the reaction to take place will define the total extent of the reaction. As was pointed out in another part of the Kiln thread this can be very quick.

The main cure to this particular problem, is to have a good temperature measurement system using good electronics with the capability to operate a good power supply that has a sensitive enough feedback loop to accurately control some sort of pulse width modulation (PWM) control device. Secondly it is also useful to know the type of metals and metal compounds being used to achieve the final result and getting some good data sheets for those materials.

Hope this helps

Karolen said...

Thank you for your comment Flying Welshman!
Oh how I wish that lampwork bead kilns were an exact science!

I've thought about the things you mention. It doesn't make sense that an electric kiln would have anything other than a basically normal air oxygen ratio, therefore I'm at a loss for how to make my kiln have a more 'reducing' environment. Others had luck with adding small bits of fishtank-type charcoal, but my charcoal just turned to dust in minutes.

There are a lot of things I don't understand about why my particular kiln behaves the way it does. It's not an exact science at the level I'm working at. This is an $800 kiln, but if I had $80,000, I bet I could find someone to build me a state-of-the-art kiln with accurate temperature controls, fully measured and tested down to the last millimeter, etc. Wow wouldn't that be a dream!

I've had to accept the fact that I'll never be able to make this an exact science. That's very frustrating when I think a + b should equal c. I've had to surrender and work with what I have in whatever way I can.

What I've done:

I noticed that the beads lost their metallic coating when they were within an inch or so of the kiln walls, floor or ceiling. I built a stand out of glass that sits inside my kiln that allows me to place beads for annealing right in the very center of the kiln, at least an inch and a half from the floor and all sides.

IAlso, I have found that the metallic finish adheres better when I reduce the beads hot. I was working the glass too cool before.

I have also reduced the temperature significantly and also the length of time that metallic beads sit in the kiln.

The combination of these things has solved my problem! Now every metallic bead that comes out has a nice finish. I still have nightmares, though. I always wonder if this will happen again!

I wish I knew the science behind what I just did with my kiln and glass. At least I can carry on now making the beads I love to make!

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