Sharp bits on edges of holes

[★★Terri★★]

Hi

I have made a some pendants and used string to make a hole.  I have got sharp pointy bits on the edge of the top layer at edge of the hole on both sides on all 3 pendants I made.

Each one was 3 layers - two were 2mm bulls eye glass for each layer, but I made a bridge over the string by cutting the central piece and setting either side of the string.  The third pendant was 3mm bulls eye on the base layer, then some 2mm dichroic and a 2mm clear bulls eye on top.

I fired the kiln to 840 c.  No fancy firing ramps as it is a hobbytec ceramic kiln - just set the temp and let it go.  It switches itself off and cools nice and slowly as it is brick lined.  It works perfectly for fusing (at least for small items like cabs and pendants).

The other edges on the pendants are nice, and even the base layer under the hole is nice and smooth - it's just the top layer that is sharp and pointy.  Nothing else in the firing is sharp.

Is the problem caused by the string or something else?  Does anyone have any ideas?

Many thanks

Terri

[treacle_mel]

Hi
I had similar problems when I used the string for holes in pendants and had to go back to rolling up kiln paper for the holes - I've never had a problem with sharp bits when I use kiln paper, but it's a bit more work.  I'll be interested if anyone has any ideas to solve the problem with the string as it would save a lot of time... perhaps it works better with 6mm glass rather than the thinner jewellery glass.

Sorry I couldn't help....

Mel

[Margram]

Bah - I've just bought some string to try.

[Zeldazog]

I sometimes get these when I use 3mm ceramic-fibre paper - I can't remember if I got it with rolled up thin-fire or not, been a while since I did it that way.  I have some rope but I haven't tried that yet.  I often reshape my pieces and re-fire (fire polish) them, so its not such an issue for me, although I have removed them with a sickle stone too - that depends on the glass you've used as to whether it will show any dullness where you've used it.

The needling could be caused by too hot a firing though; 840 seems very high for Bullseye - I full fuse float glass at that, but Bullseye I usually do at 804 in my kiln.  I know everybody fires differently and every kiln is different, I know some only go to 795 and some to 814, but yours seems very high to me.

[bluefairy]

I often find this happens with the rope/string too    I often grind cabochons afterwards as well, or just use rolled up kiln paper instead

[★★Terri★★]

Many thanks for your thoughts and input. 

It does looks to me as if the glass has just snagged onto the string while it has been shrinking/moving - so I will try paper instead.  I have some Thinfire, I'll try rolling that into little coils, or perhaps I'll chop up a mandrel and use rods with bead release.  Though thinking it through a lower firing temp would cause the glass to move less....hhhhmmmm!

Am new to fusing so still experimenting a bit with the temperatures.  The temp readout is purely what is happening where the thermocouple is and only a guide.  Working from my knowledge of firing glazes I am guessing that as the kiln is firing very fast I will have to nominally 'overfire' on the temperature to reach the eutectic point of the glass.  Firing a kiln on 'temperature' rather than 'heat work' is always a bit hit and miss. 

I had put these pendants in an experimental batch with varying results.  On the postive side I have made some lovely little cabs from a variety of glasses which I can use for tack fusing experiments.

Well, at least I can grind these nasty bits off and pop them back in the kiln.

Many thanks

Terri

[treacle_mel]

I find that the thinfire paper rolled up to expose the softer side leaves a really nice finish on the inside of the hole which is useful if the top layer is transparent.

Mel x

[Flippopotamus]

A couple of you mention grinding & fire-polishing....  my latest acquisition in my shedio is a grinder (whoopee!) and I've ground & polished quite a few of my earlier efforts.  But I've not tackled any made with the thinfire rollup or fibre rope method. 

So I was wondering - do you replace the fibre paper/rope with a new piece for the fire polish, or do you generally get away with it without the hole collapsing?  (I'm thinking that, though it's higher than slump temp, you might get away with it because it's such a small area...? ? ?)

BTW I agree that 840 is way too high for firing Bullseye, and I've read on other forums that overfiring can create the needling effect that you describe.  Even allowing for the fusing method you describe above, I'd try knocking 20 degrees off.

Cheers,
Flip

[★★Terri★★]

I am going to grind the sharp bits and refire them this week.  I have a tack fusing firing to do so will put them in with that.

Will try again at a lower temp tho.

Just to be on the safe side I will put a little coil of thinfire into the holes just incase they collapse.

Terri x

[Zeldazog]

So I was wondering - do you replace the fibre paper/rope with a new piece for the fire polish, or do you generally get away with it without the hole collapsing?  (I'm thinking that, though it's higher than slump temp, you might get away with it because it's such a small area...? ? ?)

I usually use 3mm ceramic fibre paper, and I usually cut a new piece to slot in, just in case - as you say, its higher than a slump, so although its a smaller area, remember its also half thickness, so might droop early.