Grinding Marks - again

Zoe · September 07, 2009, 07:57:10 PM

Hi there,

New to all this and hoping that you can help. I have read and read and read, oh yeah and I have read about getting rid of grinding marks and mine will just not disappear!!!

I read somewhere that it might be the thin fire causing it, has anyone else had the same problem.

I have cleaned and cleaned the cabs, tried different firing schedules but no luck.

I really am at a loss.

Please please can someone help before I throw everything in the bin, me an all!!!!

Thanks

Zoe

bluefairy · September 07, 2009, 08:00:14 PM

Hello, what kiln have you got, and what temp are you firing to?

Zoe · September 07, 2009, 08:07:17 PM

Hi Helen

I have a paragon sc2 and I have tried various temps up to 780c

Hope you can shed some light on this for me.

Thanks Zoe

Hotglass28 · September 07, 2009, 08:45:51 PM

What are you using? flat lap or bit grinder? and what grit are you going up to?

Zoe · September 07, 2009, 09:29:50 PM

Using a grinding bit and I use a standard and fine (220) grit.

julieHB · September 07, 2009, 10:17:23 PM

I was tearing my hair out before I finally found what was the key factor, and what gives consistent result every time, namely a very slow ramping up. Unfortunately I cannot find my own schedule at the moment (eeek, I thought it was programmed into my SC2, obviously I must have overwritten it), but here's an example for fused bullseye cabs (segment, ramp, end temp. and hold time:

1   175 C/hr 765 degC   10 min
2   Full    515 degC   15min (for big pieces hold longer to anneal)
3   65 C/hr 371 deg C   0min

If I want the edges more rounded I go up to 800degC between segment 1 and 2 above and hold for 0min.

I also found that as long as I had a slow ramp up up to 520degC I could go faster afterwards (that was me being impatient).

I know that some people swear by finer and finer grinding bits, and some by special detergents. I cut using a taurus ringsaw, rinse the cab and fire polish. Not a hint of a milky smudge around the edges. I only grind if I want a very smooth straight edge, and that has only to do with the desired shape, not for fire polishing purposes.

Obviously kilns are different, but I firmly believe it is the slow ramp up that is the important factor.

HTH!

Zoe · September 07, 2009, 10:24:07 PM

Thanks, can you tell me if you use thin fire?

Regards Zoe

julieHB · September 07, 2009, 10:27:24 PM

I have used both thinfire and fibre paper. Thinfire is much nicer, as it doesn't curl up around the cabs.....but it is also more expensive, and I don't have any left..

Zeldazog · September 07, 2009, 11:11:51 PM

Quote from: julieHB on September 07, 2009, 10:17:23 PM
I know that some people swear by finer and finer grinding bits, and some by special detergents. I cut using a taurus ringsaw, rinse the cab and fire polish. Not a hint of a milky smudge around the edges. I only grind if I want a very smooth straight edge, and that has only to do with the desired shape, not for fire polishing purposes.

Obviously kilns are different, but I firmly believe it is the slow ramp up that is the important factor.

HTH!

That must be the key Julie, I haven't had any problems with grinding marks - the last lot I did, I only used the 170/180 disk, no higher, and I only took the kiln to 735 or 740 (very naughty, must keep a firing log) as I was fusing some bails on to some pieces.

But my standard fusing program is usually ramp at 166 to 677, then the bubble soak, the AFAP up to process temperature. I know that it's conservative, but I honestly can't be bothered to keep changing the programme all the time I may have done a large plate or thick panel, so it covers all bases that way), so usually I only change the process temperature and soak time - no harm comes from a slow ramp or a bubble soak (even if its not needed, its harmless as it is below the devitrification zone)

julieHB · September 08, 2009, 08:17:11 AM

Dawn,
I think the numbers 166 and 677 comes from the Bullseye firing schedules for bigger slabs (the numbers ring a bell), which covers all of the various programs (fire polishing to fusing to annealing). I had a brilliant printout of these, and have tried to find it on my computer, as it showed all the different temperatures/ramp ups, when to fire and cool quick/slow. With small cabs one can get away with rushing the programs, but, as you say, it doesn't harm to heat up conservatively!!

Now, where could I have hidden that file - argh!!

I was going to mention earlier, but forgot: Zoe, I don't think the smudges stems from the Thinfire paper, it is the dreaded devitrification, which occurs at temperatures above 700 deg C. Therefore, glass should be held as short as possible at these temperatures. So, with Dawn's input as well I would probably have put in an extra segment above, heating slowly up to 677 deg C, and then fire on full up to 730-760 deg C to fire polish.

There are sprays that can be used to minimize devit, but I have never used any. In my book it says you can make your own mixing Borax and water.

Zeldazog · September 08, 2009, 08:54:32 AM

Julie

I got that original program from the BE technotes I think - it's the second ramp in plate slumping program. First rate is something like 222 or might even be 333 (will have a look see), then of course the second firing should be slower.

I know its overly conservative really but as I generally put my kiln on in the evening, or before I go out for a day, I can avoid the impatience factor. Of course, if its pendants, and I am in a hurry, I have been known to whack it up at 333 C/hr, and flash vent it after the soak at 800, so I am not particularly patient myself!

Zoe · September 09, 2009, 10:06:16 PM

Thanks very much for the advice, I am just waiting to get my workspace sorted (having a summerhouse) and will have another go with your advice, I will let you know how I go.

Thanks again

Zoe

Zoe · October 18, 2009, 09:46:15 PM

I just wanted to say THANK YOU THANK YOU THANK YOU. I have successfully fire polished a set of cabs and no milky marks just sparkling cabs. The edges were perhaps not quite as rounded as I would have liked but I am not complaining.

I was just about to sell everything! Not now.

Zoe xx

bluefairy · October 18, 2009, 10:13:50 PM

Yay, well done, don't you love it when a plan comes together

piccies please!!

julieHB · October 19, 2009, 10:23:42 AM

That's great news, Zoe!! I think I had the same feeling when I finally figured out how to get rid of those nasty smudges. If you want to round the cabs more just run through the same program but take them up, say to 800-810 deg C, and hold until they round off nicely before cooling down.