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Author Topic: Problem with drab surface on coaster  (Read 1650 times)
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Andy Davies
Forum Member
Posts: 58

« on: December 23, 2015, 08:31:03 AM »

Hi folks I wonder if you can advise me on a quality issue I have?

I have been making small glass panels using  either window glass or Bullseye glass without any significant problems but just recently I've processed a couple of coasters made with a 3mm layer of coloured and transparent glass and covered with a 3mm X 100mm X 100mm sheet of clear glass, (smooth side up.)
I only process them one at a time in my home made kiln thatís fitted with a very good programmable temperature controller.

As it happens, because of the assembly of the glass I had to re-fire some coasters, some of the joints on the back of the bottom layer were not fully fused after the first firing.

The surface finish of the glass after the first firing was everything that youíd expect, a bright smooth surface that really reflects the light. But as I say for mechanical reasons I wanted to re-fire some of them and after the re-firing they have a drab surface finish that really spoils them.
Having just looked at then again in bright sunlight, (yes the sun was actually shining over here in Wales for a while today,) I'm pretty sure that the problem is finger grease on the surface that must have been there before the start of the re-firing, I can just about make out fingerprints.
I must admit that I never take particular care over cleaning the glass before fusing and I havenít had problems before so I think that the problem seems to be associated with the re-firing.  I often get the work out of the kiln when itís too hot say 50 degrees C and I handle it to cool it down, I never was very patient, and I'm wondering if thatís when the finger grease got hardened onto the surface and then cooked in during the re-firing?

My other thought was could this be the start of devitrification promoted by the finger grease?

The first firing was a ramp of 350 deg C per hour up to 680C with a dwell at 680C for 10 minutes then up to 774C at 350C per hour with a dwell at 774C for 10 minutes followed by a 800 deg C per hour ramp down to 515 deg C for a twenty minute dwell and annealing down to 380C at 120 degrees per hour.

The second firing was as above but this time I took it to 810C with a 17 minute dwell at 810C whereas it was 774C for 10 minutes, so 36 degrees hotter for a bit longer. This cured the mechanical 

The coasters are ~6mm thick.

Itís unfortunate that I should get this problem now as these are the first coasters that Sophie has made and my insistence on re-firing them has rather taken the shine off them. As Iíve already said, the surface finish was very good after the first firing so itís something Iíve done in the second firing.

I'm going to see if I can polish then up with some tin oxide but Iíd be interested to hear you comments and experiences.
DSCF1228 by Andy Davies, on Flickr
Above. Coaster is fine after the first firing

Below. After the second firing the surface is drab

DSCF1239 by Andy Davies, on Flickr

      Merry Christmas and Kind Regards to everyone. . .  Andy


Andy Davies
flame n fuse
Forum Member
Posts: 464

« Reply #1 on: December 23, 2015, 11:15:30 AM »

Dear Andy
Good to see you're posting again. How frustrating! It could be a problem with glass cleanliness. Also, when I ramp down, I go as fast as possible down to 482 and then at 65 per hour down to 371. Perhaps dwelling at 515 for 20 minutes might have kept you within the devit zone of temperatures. The bullseye glass technote 'heat and glass' is very useful, though I often work a bit cooler than they suggest. Also some of their conversions from F to C are wrong in some of their documents! In on eof their docs, bullseye suggest that you can mask devit by using a layer of fine powder, but I've never tried it.
best wishes
Pat from Canvey
Only a little bit odd
Forum Member
Posts: 1222

Keep on blowing

« Reply #2 on: December 24, 2015, 03:04:31 AM »

Have a read of I've used a home made spray in the past when I suspected I might get devit when using beer bottle glass. It's also much cheaper than buying the made up spray. I've found that the blue glass from Harveys Bristol Cream bottles is particularly susceptible to devit.

Andy Davies
Forum Member
Posts: 58

« Reply #3 on: December 24, 2015, 04:49:19 AM »

Thanks Pat and Julia for the feedback.

   Pat it sounds like you have a good excuse for emptying a few bottles of Bristol Cream over Christmas, practice makes perfect.
      If I get around to reworking the coaster I'll post the pictures.

                        All the very best  . . . Andy


Andy Davies
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