Learn, Share, Improve

General Category => Projects => Topic started by: Administrator on January 25, 2019, 06:10:05 PM

Title: Opal Phosphate Batch
Post by: Administrator on January 25, 2019, 06:10:05 PM
A little history...opal phosphate (Milk Glass ( became interesting to me as a non- or less toxic means of achieving an opaque glass color. Since I started using enamel white I always wanted a tank to just gather a nice layer of color-contrasting white. Opal phos seems a great option for this, but it does have some drawbacks.

Opal phos can be made very dense by increasing the phosphate % and can be white drawn from the pot. However, the more phosphate the harder it is to melt. It's a little bit textured, as is, so anything that resists melting is going to make it worse. When the phosphate level is made more moderate, the phosphate strikes white as the glass cools. Heating and cooling increases the density of the white. Why? Tiny phosphate gas bubbles are formed which block light transmission. More cycles= more bubbles. Wow, right? Another issue: it is very hard to predict its compatibility in a batch calculator. Gotta test and test and test. I also wonder if the expansion changes based on the degree of gets more stiff so chances are it does.

So, I've been working with Dave Bross' formula which he formulated for electric element furnace melting. Works very nicely but I wanted it to match Cristalica cullet. Took a while but I got it matched with a theoretical LEC of 87. Now, I am trying to modify the formula to use feldspar (custer) instead of alumina hydrate. I'm also lowering the alumina to soften it up a bit and help the melt.

Future modifications for the glass will be:
- reddish tint with Black Tin
- silver-luster opal phosphate (reduced glass body/black tin)
- colors of opal phosphate (blue, purple, green, jade, red)
- Chalcedony/opal phos fusion.

Another reason for moving to a new base glass slightly different from Dave's original was to remove the nitrate. This should help in overcoming the oxidizing properties of the glass and with the help of black tin...make it neutral or reducing.

More to come...I'm actually waiting for the cryolite to arrive in the mail.

About the attachment. This is from a great website built for glaze recipe record-keeping. Lots of features I've yet to use yet but I'd been looking for a way to add comments to batches and help in making tickets to take to the batch room. It's cheap, feature-rich and easy to use. It's a user side to a public information website called

Title: Opal Phosphate Batch
Post by: Administrator on January 26, 2019, 11:09:51 AM
As I continue on this project thread, the cryolite did not show up yesterday so I made a modification to the batch in an effort to move forward.

What is Cryolite anyway?I hadn't thought of it as a batch ingredient until Pete recommended that Jordan Kube use it to help in melting his high phosphate (5%) opal batch. Cryolite is one of these naturally occurring compounds with the formula: Na3AlF6. So some sodium, aluminum and fluorine. The fluorine is what helps in the melt. Lots of the recognizable intense opaque glass colors are fluorine-based. It's an interesting element. The most highly electronegative and reactive. I've been using hydrofluoric acid in the finishing process of finishing glass. It eats glass. Even at the relatively low concentration I use it at. I'll discuss this process at some point...I think I have found THE best source for this etching material.

At higher concentrations (>4%), fluorine forms crystals in the glass that prevent light transmission. Below this level, fluorine can be used to help a glass melt. According to Weyl in Colored Glasses, it interacts with and disrupts some of the Si-O-Si bonds. This somewhat weakens the structure, lowering viscosity and in doing so helps melting. He even mentions that it has been used with great effect in making opal phosphates by preventing surface formation of apatite crystals. They form at the surface because the process requires water provided by the atmosphere. In any event, this causes the surface to become rough. My future melts will likely contain cryolite (~.5% w/w) to avoid this issue, but when the glass is encased this really becomes a non-issue.

I'll need to read more to understand the contribution of fluorines to glass expansion. Cryolite contains the following composition of oxides:
Yet, I have a calculator which estimates expansion and does not take these oxide contributions into account. When added back to the calculations, the sodium alone can create quite a discrepancy. My guess is that the fluorine makes the other components so volatile that they gas off and do not contribute to the final glass. However, this will take more reading. Fluorspar contains a similarly unattributed oxide in CaO at 59.6%. Again, very high but in combination with fluorine perhaps lost during the melt. Interesting...

I'll post on fluorine batches soon and discuss the way the calculator estimates their expansion. Pete points out that 96 LEC compatible fluorines come in around 72-75 in his estimation. We'll look at the expansion contribution a few different ways and see if a best estimator can be determined. Of course it always comes down to real world testing!  ;D