float glass slumping - Nabertherm Kiln/Kaiserlee board

[chas]

hi, forum newbie here... though I've made quite a few reasonable bowls in shop-bought moulds, so not exactly a newbie slumper, I'm stumped by this:

I'm trying to make a small rectangular plate (9" x 6") in 4mm float glass with two indents about 3" dia and 1/2inch (say 15mm) deep. I've cut a 10x7 piece of KL board through to half thickness (ie gives a drop of about half an inch) and cut out my indents right through, then placed it on Thinfire paper in the bottom of the kiln.

So far, in 4 firings on varying schedules, I've failed to get the 'indents' dropped through to give a flat-bottomed finish - they stay resolutely in rounded-bottom form. I have cut a channel on the underside of the board to make an airvent.

Are there additional stresses incurred in double-dropping that make it particularly difficult? Can't see why there should be...
Can anyone suggest a schedule, please?

Chas

[GaysieMay]

I'm not really much of a fuser, but on my bought molds they have tiny air holes, could it be that you need to let air escape so as to fill the indents?  Just a thought. 

[chas]

Thanks for the speedy and encouraging response, GM - but I thought of that after go No. 3 and cut air escape channels into the underside of the slab - and cut another mould without as a comparison. Firing No. 4 has resulted in similar rounded results to earlier firings and with no real difference between the two moulds in the same firing... if that makes sense.

I have strayed beyond 'normal' programmer settings, altering time and heat but feel I could experiment forever before I blunder onto a schedule that works.

Unfortunately, the issued Nabertherm schedules are for Bullseye, and generally increasing the temperatures across the board by 10deg work for slumping, fusing etc. This time they don't. I've cranked them up by an extra 30deg, topping at 752c.

I guess I'll just carry on into fusing temps and see what works.

Chas

[Pauline]

I don't know if this is any use
http://www.frit-happens.co.uk/forum/index.php?topic=25941.0

its a link to a previous thread about temperatures for float glass

again I'm not much of a fuser so can't really help

[Moreton]

Hi Chas, welcome to the forum.

Try a deeper "hole" it maybe that there is not enough room for the glass to reach the bottom of your mold. Also it may be too small a hole, have you allowed for the thickness/bulk of the glass?

Let us know how you get on...........pictures???

Regards Pete

[chas]

Hi Pauline, Pete, I've followed some links (always interesting and informative) and have come to the conclusion that for the sake of an overnight firing and a few penceworth of glass, I'll try a slump schedule hotter than before (meant for 2x 3mm Bulsseye fused plates slumped into a bowl) that I found.

A deeper hole isn't really an option - I want this to finish about 1 cm deep. It's typically perverse, ain't it... I've made plates with a rim before, using a stainless steel plate as a mould, depth about 1cm: no problem and beautifully flat and plate like.

So, fingers crossed and it's take 5.

I'll try some pics after tonight's try - hopefully they will show the early failures and now, success!

Chas

[dinah46]

Going hotter isn't always the best/only way - have you tried leaving it longer at slumping temp.  When I'm fusing normal shaped float glass plates in ceraboard moulds I leave at slumping temp for at least 30 minutes.  As you seem to be willing to do further tests then maybe you could extend the slump.

[chas]

Okeydokey, having strayed considerably from earlier successful 'recipies' (encouraged by you lot) firing 5 was the one. For anyone with the same kiln/glass combination, I offer it below. The only downside is that due to the Kaiserlee surface texture (I wanted texture - I know I could have sanded it smooth) around the edge were several little sharp teardrop 'drags' where the glass had grabbed the surface as it shrank a little to allow the drops. No big deal with the grinder.

So, thanks for the interest, and slower and hotter was indeed the answer. Whether dropping through multiple 'holes' causes a problem I still can't be sure. Sometime, I am going to try the reverse - draping a sheet over similar shapes -  though the expected dull surface may need to be fire polished (I don't really want additional process) and I do want the texture on the eventual underside...

The kiln works on "time to achieve temperature" not "rate", so you'll have to do the math(s) if needed for yours:

4hours 20mins up to 480c, hold 20mins
asap to 835c, hold 20mins
asap down to 515c, hold 1 hour
1 hour down to 371c, then freefall cool to end.

Chas