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Modeling Glass FAQs

I mixed the Modeling Glass wrong. What to do?
It’s important to measure accurately when mixing Modeling Glass. You need to combine the ingredients in the designated order. However, accidents happen, so here are recommended fixes for common errors:

What if my Modeling Glass is too runny/sticky?
If you accidentally added too much water or not enough Powdered Binder, the consistency of your MG could be too runny or sticky. To correct this, you can leave the MG exposed to air for several hours, kneading it periodically until some of the water evaporates (if it’s only a little runny). If it’s very runny, you can try adding a small amount of additional glass powder to create a drier consistency, though the MG will become sticky if you add too much. Adding a few drops of Liquid Medium will help correct the stickiness. Thin MG works great with a palette knife, so all is not lost!

What if my MG is too dry?
If you want the MG to be softer/moister, simply add a little more water to the ball of material using a sprayer and knead it in thoroughly. If you want it to be a lot moister, continue to add water until you achieve the desired texture. The MG will become stickier as it gets wetter, so allow for this. You can add a few drops of additional Liquid Medium to correct some of the stickiness.

What if I added too much Liquid Medium?
The Liquid Medium will burn away during firing, but if you added way too much, the MG will not hold its shape. In this case, try adding more glass powder to the MG, kneading it in thoroughly.

Can I add more Powdered Binder to fix consistency problems?
Because it’s difficult to get additional Powdered Binder thoroughly blended into an already-mixed batch of MG, adding it after the initial mixing is not recommended.

I mistakenly added the Liquid Medium to the glass powder before the Powdered Binder or water. What now?
Because the Liquid Medium affects the way the powder will react with the binder and water, it’s unlikely you will be able to get the MG to a proper consistency. Consider using it as a paste with a palette knife instead.

A special note about working with white:The best white is achieved using Bullseye Opaque White 0013-0008. A much longer hold and a slightly higher temp matures the white much better. The photo on the right has 3 petals: the one on the left was fired the usual schedule, up to 1275 for 10 minutes. It's decidedly gray. The middle one was fired only once, peak temp 1325 and held for one hour. The petal on the right was fired once to 1275/ 10 min. hold, once to 1275/ 1 hour hold, then once to 1325/ 1 hour hold. You can see the petal on the right is much brighter than the one on the far left, but only slightly brighter than the middle one that was fired once. The slightly higher temp of 1325 with a 1 hour hold is the right approach. If you are after the super-white look, then try making the hold even longer. Loss of detail in the texture was minimal even at 1325. 

I have some CMC…can I use it instead of the Powdered Binder?
The ingredients in Modeling Glass have been painstakingly researched and sourced for purity and strength. There are many grades of CMC for different applications; for consistent results, it’s best to use only the powdered binder in the Modeling Glass kit.

Is the Liquid Medium the same as Glastac?
No, it is a completely different substance. Glastac is not intended to mix Modeling Glass.

How can I get my Modeling Glass shapes to dry faster?
If you have some shapes on a nonstick mat and want them to dry faster, dry them in your kiln at 200 degrees F. After a couple of hours (depending on the size/thickness of your piece) you can gentlylift the piece away from the mat and turn it over. This will help the reverse side dry completely.

I dried my shape then broke it during handling. Can I fix this?
MG shapes are very easily repaired if broken before firing. Simply take a small bristle brush, dip it in water, and wet the two broken edges. Press them firmly back together and let dry thoroughly. You can also add more wet MG to an already-dry shape without any problem. Just be careful handling it, because the moist area will now be a weak point until it dries completely.

Can I sand dry Modeling Glass?
One of the great features of MG is that it can be easily worked when dry before it’s fired, greatly reducing the amount of coldworking needed after firing. It may be sanded, carved, and even re-moistened for adding texture. Be sure to wear a mask or respirator when sanding MG.

Will Modeling Glass colors react between each other?
It depends on the temperature. If you fire MG to a full-fuse temperature, the colors will react with one another just as they would with any sheet glass (copper vs. sulfur, etc.). See your Bullseye Glass color guide for reactions between colors and plan accordingly. If you are only firing to tack-fuse temperatures, there will be no reaction between the colors.

Does Modeling Glass maintain its COE/compatibility with other glass?
Yes. The COE or compatibility of glass powder is not affected by mixing with Modeling Glass.

Can I fire Modeling Glass on a washed kiln shelf or do I have to use shelf paper?
MG can be fired directly onto a properly prepared kiln shelf, and it works fine on shelf paper as well. Be sure to thoroughly clean kiln wash or shelf paper residue off the back of the fired piece.

My Modeling Glass piece shrank when I fired it!
One tenet of all shapes made with powders: Shrinkage Happens. You can count on shrinkage of 10-15% from its dry size. Because powdered glass sinters as it heats, the particles melt together and lose volume as air is forced out and the binder burns away. The resulting shape is pure glass, and depending on the amount of heatwork, may still have a small amount of porosity, or it may end up very glassy and solid. The hotter you go, the more sintered (solid) the shape will become.

How can I control shrinkage in my design?
The easiest way to address shrinkage in the elements of your design is to build the elements separately, and pre-fire them to a low tack-fuse temperature (1275 peak with a five-minute hold). This will allow the pieces to shrink, then you can combine them into the final design. MG pieces are surprisingly strong even before firing, so it’s not difficult to sculpt and fire them, then work with individual elements in this way. It’s also easy to cold-work the pieces after firing, before placing them into your design. If the piece you are building is all MG (no sheet glass involved), it will all shrink together, so you can build the entire piece without worrying about uneven shrinkage.

Important note: dark colors like black will absorb more heat and shrink more than lighter colors, so factor this into your design process.

My Modeling Glass piece lost all its textural detail after firing.
To preserve the detail in your shapes, it’s recommended not to go above 1300 peak temperature for 10 minutes. Going above that temperature will cause the piece to be more like a contour-fused shape, with softer edges and lost detail.

Are Modeling Glass ingredients toxic?
The Powdered Binder and Liquid Medium are both food-safe and nontoxic materials. In fact, the Liquid Medium is used in cosmetics, and the Powdered Binder is used in food preparation.

What happens when the binder burns away during firing?
As with firing any material that has a binder, it’s recommended that you vent your kiln until it reaches 1000 degrees. The binder and liquid medium burn off between 800-900 degrees. You may smell an odor that is like hot cardboard—that is the cellulose-based binder burning out. It’s recommended that your kiln be located in a well-ventilated space. If you dislike the smell, wear a respirator when checking the firing.

The colors didn’t mature in my fused piece.
If you are firing to tack-fuse temperatures, some colors will not be true to their full-fuse temperatures. This is particularly true of white, reds and yellows. Cumulative heatwork will result in brighter colors: in other words, the more times a piece goes through the kiln, the brighter the colors will become as the powder sinters more…even if you are not going higher in peak temperature. Firing MG at full-fuse temperatures will result in true final colors.

Will striker colors strike with Modeling Glass?
As with regular striker sheet glass, striker powders will not mature until they are full-fused. Therefore, tack-fused MG striker glass will not mature to its strike color.

Can I order larger refills of just the product I need?

Yes! The Powdered Binder and Liquid Medium can be purchased separately, ask your fused glass supply retailer.