Needling is a description of the fine points emerging from the edges of glass.
This occurs in two conditions mainly.
The one that is most commonly seen is in the fusing of single layers of glass. The surface tension of the glass pulls the glass in from its original size, trying to achieve the 6-7mm that is a thickness equilibrium at full fusing temperatures. If the surface the glass is resting on has any rough areas, and most surfaces do, some of the glass will stick and the rest retract. This leaves short, thin and extremely sharp “needles” extending from the edges.
Two common surfaces allow these sharp edges. Fibre paper of 0.5mm and greater is rough enough to allow the hot glass to stick to tiny depressions in the paper. Kiln wash is often not smooth enough to prevent this kind of sticking either. You can smooth powdered kiln wash or aluminia hydrate over these surfaces to reduce the grabbing of the surface by the hot glass. However, the powder is often drawn back with the contracting glass. Thinfire or Papyros paper is fine enough to avoid the needling most of the time without any addition of powders.
The other main condition is in casting, mainly box casting or damming. In this case, the stack of glass sheets or cullet is higher before firing than its final thickness. This means the glass flows out to the dams and sinks down to its final thickness during the firing process. As the glass touches the fibre paper or other separator it behaves just as the single layer of glass does. Some sticks to the surface while the rest is dragged away by the surface tension and reducing thickness of the stack of glass.
Prevention of Needling
Lining dams
Separators for dams
I find your tips so valuable. I am making napkin rings using a mold and find needling is happening so feel the second condition applies.
ReplyDeleteNow that it is identified, do you have any suggestions on how to prevent this happening.In order to remove the glass rings from the mold I am using fibre paper in the centre.I have read the possibility is overfiring but the rings are still very textured, not smooth. I use a fine frit.
Firing: 175*C > 740*C > 30mins
AFAP > 525*C > 30 mins.
thank you!
I am not clear about the characteristics of the mould you are using, but will make some general comments that may be of use. Let me now if they are off base.
ReplyDeleteYou will know by now how far the glass sinks into the mould in relation to the height of glass you put into the mould. So cut 3mm fibre paper to 3mm less than the final height of the napkin ring. Line that with thinfire of the same height. Do this for the inside too. The principle is that the glass will sink and spread rounding at the edges to give the "bullnose" effect that occurs over the last 3mm of the piece's edges. The bottom will be square though.
This lining method will give the least marked glass. You can then go on to give the outside a final polish by cold working if you want.
I don't know what glass you are using, but 740ºC seems low for any glass. It is at the bottom of the tack fusing range. Casting temperatures are normally in the 820º to 850ºC range.
The finer the frit you use the more "cloudy" the final piece will be because of the multitude of tiny bubbles trapped between the spaces of the frit.
You do not seem to have any annealing cool.
My firing might be something like:
320 to 830, 10' [no problem of heat shock of the glass, however if you have a ceramic mould to care for, then you should go slower to ca 540 and then you can speed up). You may need to peek in the kiln to see whether more than 10 mins is required.
You could put a soak in the "up" part of the schedule at about 650º to 670ºC for about 30 mins.
AFAP to annealing temperature. The length of soak here is related to the thickness of the glass. I assume it will be over 6mm, maybe 9 or 10mm. So I would soak for 60 - 90 minutes.
Annealing cool: 60ºC/hr to 472ºC, 0'
2nd stage cool: 120C/hr to 370ºC, 0'
final cool 250C/hr to 50ºC