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is there anyone out there who can reassure me?

i am in the process of rigidising 6mm fibre board 1000mm x 600mm
after applying the colloidal silica and drying naturally for a week
it went into the kiln at 135 c till there was no condensation on the mirror
BUT it has turned brown
this hasn't happened before but i've never used 6mm before
if i go ahead and apply the cement and dry that
will it still provide a smooth, hard surface onto which i can fire?
michelle s

Most fibre products turn brown on the initial heating, then return to white as the temperature rises. Having only brought it to 135c, you've only partially burned out the organics. Your fibre board should return to white as you bring it hotter in actual use.

Prefire to 50ºF hotter than you plan to use the board. It will return to white.

thanks for your replies chaim and bert
just to clarify
i should fire the board at around 840 C before i paint the cement on?
michelle s

michelle schouten wrote:thanks for your replies chaim and bert
just to clarify
i should fire the board at around 840 C before i paint the cement on?
michelle s

I have never rigidized a board, so I don't know what would happen if you rigidized it before you burned it out. I can see no downsides to prefiring it and then rigidizing.

I've rigidized, let dry and then cured at about 1000F (540C). No need to go higher. Big stink around 600 to 800F, ventilate well.

Do I understand the fiber board is only 6 mm thick? You are rigidizing it and then coating with some cement? What is a cement that makes such thin fiber into a board that will stand up to fusing? I have had some serious sticking on rigidized board but never made a cement coating.

6mm fiber board only comes in the LD line. What is the point of rigidizing it? It is quite weak, and I fail to see how rigidizing it will make a significant difference. Standard rigidizer is made of silica, and sticks to glass. The board itself is much less prone to sticking. My habit is to dust boards with finely ground alumina hydrate. It doesn't take much to make them not sticky. I have done this with good success for both float and Bullseye glasses.

6mm board needs to be totally supported. It does create a pretty good firing surface, and it will cover any seams in an abutted shelf system it covers. Personally I prefer using 6mm blanket for the same purpose. Both are good for many firings, if handled minimally.

marian
the cement i'm using is QF-180 by Unifrax
its a tedious process but worthwhile
i'm still using a 25mm thick shelf 5 years after i treated it in the same manner
the process i use is detailed in bullseye's technotes 6
i'm using 6mm this time because its the thinness that will allow me to use the full length of my kiln floor without being too close to elements
michelle s

Thanks for the reference, Michelle. I enjoyed reading the Tech Notes. There are more big glass kilns than ever, and big kiln shelves. I hope you are happily making lovely giant glass pieces on the new shelf.

I did once, in somebody else's shop, work on a shelf prepared according to the Bullseye tech notes. It was 1" or 2" thick, I forget. The surface was very carefully ground flat and smooth, using a hard kiln shelf as the sanding device, while constantly checking with a straight edge. Every firing required using thinfire. Personally I was not the least bit impressed enough to copy the procedure. I don't understand why they used colloidal silica to rigidize, instead of colloidal alumina. I am not the least bit obsessed with smoothness on my underside. I figured out long ago, that I wasn't going to get a flat, smooth, shiny, bottom. So, I decided to make the bottom an important design feature, with intentional texture.