Diary of a Demented Store Owner

Saturday, 17 August 2013

Fusing- Devitrification, Even Us

While we spoke earlier in this enlightened blog about devitrification (see here), we neglected to tell you what it actually is (Mikey was reminiscing about his long gone mullet obviously...Vanna Opal). ‘Devit’ is a not uncommon problem fusers will suffer from sooner or later- a crystallization of the surface of the glass which like unwanted houseguests is not easy to deal with or as attractive (for an overly simple explanation). Sadly, as in many mother-in-laws, it is not removable. Happily, however, it is treatable.

Moving along, let’s see what we can do to fix it.

As you gaze in wonderment and amazement at this inspiring work of art and skill done by Mikey and Zenia, you might notice that the SP1009SF Black base for this project devitrified badly. Oh, the horror!

But why? 
Some possible causes and solutions;

• it’s possible that the Bullseye endorsed glass cleaner caused it. This is the first time we tried this cleaner which smells like a combination of Windex and alcohol. We typically use isopropyl alcohol as a final cleanup after washing our glass with a detergent (alcohol is not very good at cutting through cutter lubricants which are mostly oil-based- another reason we prefer kerosene (yes, it also is an oil, but almost not). Remember, always handle cleaned glass by the edges with oil-free hands.
• Perhaps the SP1009SF Glass was the cause. It was fusible which means that aside from having a known expansion, it is also formulated to minimize devit, but not always and opals are more prone to devit.  Multiple firings also could encourage devit. We often purposely dirty up a small sample and do a test firing before investing a lot of time and effort using a glass that might be suspect. If we still want to use a glass that might cause us problems there are options such as capping with clear, sprinkling a thin layer of clear glass powder or overspraying with such readily available products such as Spray A and Borax. 
• Did we run a program too long in the ‘devit zone’? This would be temperatures above 1300F. The longer we keep our glass above this temp, the higher the susceptibility it will have to devitrify. Best case scenario is to fire up to between 1000-1250F with a hold of 10-30 minutes and then speed up the firing (500F or more) until you reach your terminal temperature. Of course, there are ranges here- you have to adapt your schedule according to project size and/or thickness.
• Was the ‘air’ in the kiln polluted meaning the kiln and peepholes were closed and unable to vent off any impurities (glue, shelf paper, duck fat)? Some glues are better, some worse at causing devit. One we can heartily recommend is Bullseye’s Glastac, or favourite. Minimize their use or just stop using your glass for roasting dinner...
• Grinding. Working a piece of glass with a glass grinder will almost certainly increase the likelihood of devit. Focusing on good glass cutting to remove the need for a grinder is the best solution as a smooth glass cut edge is best. Second best is to not let your ground pieces dry out by keeping them in a tub of water with some detergent, vinegar (we are Canadian, but that doesn’t mean to use cider vinegar, ok?!) or even grinder coolant to increase the water’s wetness (that’s right- coolant actually does that) and when ready to set up in the kiln, scrub all edges with a toothbrush (Mikey gets a thrill out of using Vanna Opal’s Oral B Ultrasonic toothbrush and not telling her).
• Any combination of the above. This last one is one that we like the best. You see, you might be doing your prep and firing where you are just on the edge of having devit occur but not quite. Then, some minor anomaly occurs (you peek in the kiln because you are just too nosy and impatient thereby usurping the firing schedule, or you missed a fingerprint, etc). One change might have not been an issue, but the two together might have.

So Mikey you ponder, now that I know how to avoid devitrification, how do I fix the piece I made yesterday?
One way of repairing devit involves is by removing the top skin of the glass. Sandblasting, grinding, acid etching- all labour intensive and for most our applications, a little too involved. There is a simpler and equally effective solution and that is to use some sort of top coat to coat your work and thereby 'glazing out' the devitrification. Zenia’s preferred way is to spray coat a thin layer of Spray A (or Borax or other overspray- remember, some may not be food safe as they may contain lead) using a mouth atomizer, and then refiring. Easy, uncomplicated and virtually 100 percent successful. And that's what we did to fix this piece-

In closing, keep in mind that it is more preferable to avoid devit to begin with than it is to have to deal with it after...


Doris said...

FINALLY. I thought you were never going to give me the solution!!! Well while you are holidays I will try it on all those disasters I have sitting around.

Have a good holiday and as the song goes....SEE YOU IN SEPTEMBER!

Wacky Pup said...

I'm dealing with exactly same Devit on black glass. I would love to be able to fix it. Are you saying that you simply spray it with borax solution and refire it? What temperature to fire it to? I am using 96 coe glass. Right now I have it sitting on the counter with a bunch of armour etch on it.

Thanks for any help!

Mikey Figgy said...

In answering your question, I thought I'd also paste this in to give you some added insight. But if you don't want to read it all, the answer to your ? is to fire at high enough a temp to melt the overspray which would put it close to fusing temp- safely 1400 would work without distorting the piece-
In glass fusing overglazes are applied to the glass assemblage prior to
firing in the interest of eliminating surface crystallization
>Do these use Borax as an ingredient too. Is there reason to worry regarding durability on outdoor pieces with >either of these clear
glazes? I am far more concerned with quality and durability than with
saving a couple >of bucks short term.
All frits (powdered glasses) which you might use to avoid surface
devitrification probably contained borax in the batch from which they
were made. All frits used in fusing overglazes are based on soft
borosilicate glasses, and borax serves to introduce the "boro" part, if
you will. One isn't aware that any fritmaker in the US is melting for the
sole purpose of making overglaze for fused glass. Seems like a year's
consumption for these ends would be a few hours production. Rather,
ceramic overglaze frits have been adapted to fused glass.

Commercial frits are compounded by highly competent individuals with high
chemical durability in mind. Nothing like having your frit dissolve away
in use to lose good customers, you know.

Spray "A" is (was?) Drakenfeld 61500 clear overglaze. The 615xx series of
ceramic overglazes were staple production for Drakenfeld for many years.
Of course, Drakenfeld has been through a number of changes in the past 20
years or so and the firm, now merged with Degussa, is now known as
Cerdec. In this the nature of the offerings have changed and it is
possible that the basic clear overglaze frit used for a Spray "A" sort
of product has changed, too. Probably to a version using less PbO and
more ZnO -- this would probably describe the harder of the overglazes
mentioned in a prior post. Enamels based on these and similar series of
ceramic overglaze produced by a variety of makers are all applicable to
all manner of baked glass processes.

The harder (higher firing) overglaze might reasonably be expected to
provide the highest durability -- this is a general rule, with notable
exceptions, of course. Choose an overglaze that will be sufficiently
fluid at the peak temperature required for the effects you want and the
glasses you use. What matters is that the overglaze fuses well onto the
surface of the ware. Underfired overglaze, if it contains PbO, may leach
Pb in amounts exceeding a variety of Standards of concern to people
making tableware. It is also possible that underfired overglaze will be
more susceptible to atmospheric attack than a well melted film -- there
are a couple reasons for this I'm not interested in going into right now.
It's late.

In fact, very little is known about the absolute durability of fused
glass surfaces produced by overglazing. One has seen several examples of
corroded overglazed fused glass outdoors, but really very few and most
showing poor technique in making or installation.