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View Full Version : Best practices for 'striking' colours


Randy Kaltenbach
05-02-2012, 10:13 AM
I've never been satisfied with the strike that I have achieved on 'striking' colours (yes, this is an acceptable spelling ;) ). I have tried to get pieces to strike on the pipe, and I am looking for some direction. Some starting questions (assuming, for example, I'm making a 12" vase or 12" bowl):

Is the general idea, "get the piece REALLY cold, then get it REALLY hot"?
Can I do this before transfer to punty, or must it be done shortly before boxing? I.e. how much can the struck colour be worked after it strikes?
Does it matter whether the colour is encased or exposed to the gloryhole?
If I add canes (say 3/16") of a striking colour to a non-strking base colour, would the effect likely be visible?
Are all "opal" colours, striking colours?
I have experimented with some colours, but I am trying to tune my approach. Of course, I realize that my mileage may vary.

Thanks!!

Pete VanderLaan
05-02-2012, 11:53 AM
It isn't a one size fits all answer Randy. It varies from color to color and piece to piece. The truly difficult pieces to strike are open forms, largely spun. That is because they crash through the range where many colors get their molecular houses in order slowly. The copper rubies are the hardest for this. If I were pushing on a strike for them, I would be looking for an annealer that reduces, not oxidizes. I would be trying to put the piece away hot, not cold and in a hotter annealer. I would soak it for at least three hours before starting to bring it down.

My silver glasses perform better as a pot glass than as a rod and that is true of a lot of colors using big colloidal suspensions, or even crystals to make their colors bloom. The silvers need to be picked up, gotten hot, blown out a bit, then really cooled and heated middle hard again. It may be a wash rinse repeat to get the most color. From the pot, I just cool them and then case in clear and they bloom. The rod is harder to work with but it does work.

Fluorine opals are pretty much crystal formulations of calcium or aluminum fluoride crystals in the host glass and will strike without any special treatment. You just need to be clear on how much you need for the size piece you're working. Those glasses are more troubling in the pot than in the rod since the crystals grow, collapse and grow again until they have passed their prime and begin to lose opalescence.
Phosphate opals are relying on forming a phosphate glass emulsified inside of a silica glass. The phase separation occurs when the glass is heated and cooled and heated again. It can be denser in areas where it has been cooled unevenly, like from a ribbing mold. Those glasses do best in pot draws again. I haven't seen phosphate opals I consider to be effective in rod form. Way back Zimmermann was making some great mother of pearl phosphates but they were not compatible with any 96 glasses.

The enamels are a different animal being lead arsenate glasses with about 6% arsenic. Totally opaque and dangerous to your health without ventilation. The sweet smell is arsenic. It kills mice.

The toughest ones are really the cad sel reds which can switch valence on you in the gloryhole. Selenium can be in a lot of valence states and each one shows a different tone which is critical to the red looking good in your piece. A good red can be ruined on the blowpipe. Avoiding reduction or high temperatures helps a lot there but prolonged working of those glasses makes them muddy. They do well in slow soak environments with elevated annealing temps.

I hope that helps. It's a gloss over of how I've spent a lot of my life. I'm sure Hugh or Dave or Kenny will have other helpful thoughts.

Hugh Jenkins
05-02-2012, 04:41 PM
The best copper rubies I ever used actually needed to be reannealed at least once. But the pieces that went into the annealer early in the day were more colored than those late in the day, so soak time can be a big factor. We once made a two foot tube of copper red, cut it into 1" beads and put them in a bowl in the corner of the annealer and took one out every day. It took a month of school days to get through the whole series but they continued to change for the entire time. The best red was about four days in. After that they started to cloud up and in the end looked like red terra cotta brick. I still have most of them and can line them up by striking history easily.

The silvers are such a crap shoot, but you almost always win something. The furnace setting is quite critical, and silver opals can be light and transparent to almost black with crazy color streaks.

Dave Bross
05-03-2012, 05:15 AM
That pretty much sums up all I know.
Mostly because other than what's turned up in a number of experiments these two guys and Jon Myers taught me most of what I know about striking glasses.

As far as reds...I'm going to post this again because it's the best summary I've ever seen on reds and it has (most of) the answers on the annealing = color issue with the copper rubies. This old post points to the specific page for that. If you go to download it the link is way down at the bottom:

http://talk.craftweb.com/showpost.php?p=97414&postcount=10


While we're posting Glafo links and talking reds, here's one on gold ruby.

http://www.glafo.se/eng_ngf/previous_programmes/previous_programmes.htm

Scroll down to:

Annual meeting in Copenhagen, Denmark, 2005
Gold ruby an environmental friendly red colour (pdf)

Lots of other good reading there too.

I'll have a go at the specific questions too:

Is the general idea, "get the piece REALLY cold, then get it REALLY hot"?

Just really cold will usually do it on the reduction striking glasses. Always experiment with different heat sequences though. Serendipity ...
Opals will "go" almost no matter what.

Can I do this before transfer to punty, or must it be done shortly before boxing? I.e. how much can the struck colour be worked after it strikes?

Each one has a different temp on the cool down for the strike so you get to "tease out" the strike. Some keep striking no matter what until they self destruct. No stopping them. The temp. going up and down is the main thing...let it get too cool and it will strike harder on the reheat.

Does it matter whether the colour is encased or exposed to the gloryhole?

Depends on the formulation. enough built in reduction (black tin usually) on reduction strike glasses and it doesn't matter. The temp. is the major player. A reducing clear can really up the reaction. A reducing glory can also up the reaction. This is just for the tricky ones like the silver glasses. See comments above for cad sels and the opals are going to strike almost no matter what, assuming they cool off at least once. Usually they don't have to get very much cooler than when they first come from the furnace so, as the Borg say, "resistance is futile". Repeat strikes on the opals will usually up the density and sometimes move the expansion around.

If I add canes (say 3/16") of a striking colour to a non-strking base colour, would the effect likely be visible?
Are all "opal" colours, striking colours?

If there's enough reduction formulated in to a glass that strikes in reduction they will "go off" if the temp. sequence is followed. Stretching out a striking glass (blown out/canepulled) tends to decrease the visibility of the strike goodies a bit.

All the opals I can think of are striking.

Randy Kaltenbach
05-03-2012, 10:07 AM
Again, gentlemen, thank you for your thorough answers!!