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White scuzz on surface of open faced kiln cast

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White scuzz on surface of open faced kiln cast

Postby library » Wed Jun 19, 2019 9:57 am


I am writing to ask for help in solving a problem. I am currently doing open faced relief sculptures, some of which are showing a haziness upon the glass that is in contact with the mold itself. I am following the bullseye method with certain deviations (using silicone or wax positives). I am cleaning the glass using the bullseye method. I am using 001841-0065 Spruce Green Billet. The works are placed in my kiln on stainless steel bowl full of sand.

I have air dried the mold for a month and then placed the empty mold in my kiln at 200f for 8hrs. Then, after weighing and cleaning the glass, i place the glass in the mold and fire at the following schedule:

1: 100f - 200f - 4:00hrs
2: 100f - 1250f - 2:00hrs
3: 600f - 1525f - 1:30hrs
4: 9999f - 900f - 4:00hrs
5: 7f - 800f - 0:00hrs
6: 12f - 700 - 0:00hrs
7: 41f - 70f - 0:00hrs

I have included two images. The first (IMG1.jpg), shows a successful cast, there is not haziness on the surface - the surface only reflects the surface texture of the original positive. This particular mold came from a wax positive that was steamed out. I have also produced successful molds using a silcone positive, in which the two-layered investment was created over the silicone positive.

The second image (IMG2.jpg), shows signs of a scummy like haziness. I will say at this point that there are no signs of devitrification on the back of either of these work, only a scum on the surface that was in contact with the mold.

I am, essentially, seeking to eliminate whatever is causing this problem. So far i have excluded the following:

1) It is not the glass cleaning, as i am meticulous about this, and again, the is little to no devitrification on the back - and i am told that the open surface is where devit will usually appear
2) It is not the material that i make my positive with as i have has sucess with both steamed-out wax and a peeled out silicone positive
3) It is not the firing schedule as i have had numerous successes with the above schedule
4) I have been using un-distilled water the entire time, and again, have had numerous sucesses- so it is not that
5) I have thoroughly vacuumed the surfaces of my molds before placing glass billet in them - so it is not that
6) It is not the stainless steel bowl as again, i have had success using it numerous times.

My only thoughts are:

1) The investment material might not be mixed properly
2) The investment material might be old

If you look at IMG2.JPG you will see 'water' like movements of this haziness upon the surface, which leads me to think this is either from the glass flowing over the mold OR that this inconsistency has actually come from when i applied the investments to my original positive.

What is also frustrating is that my "failed" mold (ones which show this scuzz or haziness) have patches of being "clear" like my successful mold. If you look at IMG3.jpg i have drawn a red arrow pointing to an area on the glass that seems unaffected by this scuzz.

If you have read this far, thanks - and again, i'm kind of at a loss, any help would be appreciated.


IMG1.jpg (131.5 KiB) Viewed 7582 times
IMG2.jpg (119.02 KiB) Viewed 7582 times
IMG3.jpg (66.34 KiB) Viewed 7582 times
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Re: White scuzz on surface of open faced kiln cast

Postby marykaynitchie » Thu Jun 20, 2019 11:48 am

Here is some advice from Ted of our Research and Education department.

I think that using distilled water may help, as municipal water can fluctuate in quality/content.

The fact that the surfaces are more likely to be clean at the edges and corners suggests that this has something to do with the glass being in direct contact with the mold surface while things are still coming out of the mold. I am assuming the glass had to flow into the areas that appear to be clean, whereas it was already in contact with the mold in areas where we are seeing scum.

One thing that we have noticed can make a big difference in this contact surface is to use strategies to keep the glass from making contact with the mold until all chemically bound water has been released (by around 350°F) and also after the organic material and sulfur has left the mold (much more in the neighborhood of 800-1000°F). Typical strategies include:

Firing the mold empty to somewhere in the range of 1000°F-1100°F and then returning it to room temperature to load with glass. Downside is that the mold will be much more fragile at this point than it was prior to firing.
Load the glass into crucibles above the mold so that it doesn’t begin flowing into it until casting temperature is reached.
Make 1”-2” diameter demi-spheres (like making frit balls, but use small pieces of billet or sheet glass instead), clean the bottoms of them well, set them domed-side down on the mold surface, then load the billets on top of them. Having a radiused contact surface as opposed to a flat contact surface seems to make a great difference to these kinds of surface issues. To make the demi-spheres, follow the same basic guidelines for frit balls, but start with small chunks of billet or squares of sheet glass instead: http://www.bullseyeglass.com/images/sto ... _Balls.pdf
To fire the empty mold as in option 1, I would recommend using the same up cycle that was previously used for firing the glass – since those first two segments are really geared more towards firing for stability of the mold than they are concerned with the glass.

I hope you find this helpful. It makes me proud to work in a factory with staff that helps with questions like this from users. Thank you, Ted!

Mary Kay
Mary Kay Nitchie
Bullseye Glass Co.

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