Bullseye 000403 Opaline Opalescent: A Practical Guide for Glass Fusers

Bullseye 000403 Opaline Opalescent is one of those glasses that can be a little confusing the first time you use it. In its cold, unfired form, it can look quite clear, sometimes with only a few white wisps. After firing, though, it can develop into a soft, hazy, milky glass that lets light through in a warm way.

Bullseye lists 000403 as a striker, which means the final fired appearance may not show properly until the glass has received enough heatwork in the kiln. That is the main thing to understand before using it in a project.

This article is only about using Bullseye 000403 Opaline Opalescent in glass fusing. It is written for beginner and hobby glass fusers who want to know what this glass does, when to use it, and what to watch for before putting it into an important piece.

What 000403 Opaline Is

Bullseye 000403 Opaline Opalescent is not simply a plain white glass. It is better to think of it as a soft, translucent, light-changing glass. It can make other colours look softer, warmer, mistier, or more complex, especially when it is used as a cap or overlay.

Bullseye lists 000403 as:

• A striker
• Non-reactive and neutral
• Bullseye Compatible
• Available as sheet glass and frit
• Available in sheet forms 000403-0030 and 000403-0050
• Available as frit in powder, fine, medium, and coarse forms

The sheet glass forms are useful for caps, layers, strips, tiles, bowls, plates, platters, and design features. The frit forms are useful for soft colour fields, frit balls, fading effects, surface detail, and “Painting with Glass” style work.

What It Looks Like Before Firing

Before firing, 000403 Opaline can look surprisingly clear. Bullseye describes the cold sheet as transparent and clear, with occasional wisps of white. That means a beginner could easily look at the sheet and think it does not look like opalescent glass at all.

This is normal. Do not judge this glass only by the cold sheet. With Opaline, the kiln is part of the colour development. If you want to know what it will really do in your kiln, make a small test tile before using it in a larger project.

One simple tip is to cut it over a dark surface or place dark paper under it while cutting. Because the unfired glass can be quite clear, the edges may be harder to see on a light bench.

What It Looks Like After Firing

After a full fuse, 000403 Opaline can strike to a hazy, milky white. It still lets light through, but it changes the quality of the light. Bullseye notes that thinner pieces can transmit a more yellowish hue, while thicker areas can look warmer and almost orange. More heatwork can also increase the opacity.

This is why Opaline can look different depending on how you use it. A thin piece, a thick piece, a capped piece, and a frit-heavy area may not all look the same. A piece may also look different lying flat on a table compared with being held up to a window or used in a light-catching design.

That is part of the appeal of this glass, but it is also why testing matters. If you need a very predictable result, especially for a matching set, do not rely only on catalogue images.

The Most Important Thing: It Needs Enough Heat to Strike

The most important thing to know is that 000403 may not develop its milky fired look in a low tack fuse. Bullseye says Opaline may remain clear or largely unstruck at tack fuse temperatures of 1275–1425°F, which is about 691–774°C.

If you use it in a full fuse, it is more likely to develop the expected hazy, milky effect. If you use it in a tack fuse project, it may stay clearer than you expected. That does not mean anything went wrong with the glass. It usually means it did not receive enough heatwork to strike fully.

For tack fuse projects, Bullseye says a single sheet can be pre-struck at 1470–1480°F for 10 minutes, which is about 799–804°C. The pre-struck sheet can then be cut and used in lower-temperature work.

Pre-Striking 000403 for Tack Fuse Projects

Pre-striking simply means firing the Opaline first so it develops its fired appearance before you use it in the final project. This can be useful if you want small raised Opaline pieces in a tack fused design, but still want them to look milky rather than clear.

Bullseye notes that when a sheet is pre-struck, the edges can pull in from the original footprint. For sheets larger than 10 x 10 inches, Bullseye suggests considering 1/16 inch fibre paper topped with ThinFire. This helps air escape sideways and helps prevent a large bubble forming between the sheet glass and the kiln shelf.

There is one more point to remember. Bullseye says the pre-struck sheet may have traces of ThinFire or shelf primer on the shelf side, and this can affect later firings. Bullseye also notes that the opacity may continue to develop in subsequent firings.

In practical terms, mark or remember which side was down during the pre-strike firing. If the shelf side has residue, treat it carefully before using it as a visible surface in another piece.

Using 000403 as an Overlay

One of the best uses for 000403 Opaline is as an overlay or cap. It can change the colours underneath it, sometimes gently and sometimes quite dramatically. Bullseye’s Opaline overlay guide says to expect subtle changes over light-value styles and more dramatic effects over dark-value styles.

This means Opaline is not the same as using clear glass as a cap. Clear glass usually lets the colour underneath stay more direct. Opaline changes the way that colour reads. It can soften it, warm it, mute it, or give it a misty glow.

This can be very useful in fused glass design. It can also surprise you if you expected the base colour to stay the same.

Good Ways to Use Opaline Overlays

Opaline overlays are useful when you want a softer or more atmospheric look. They can work well in:

• Misty landscape pieces
• Cloud and sky effects
• Water effects
• Soft abstract backgrounds
• Jewellery cabochons
• Bowls, plates, and platters where you want depth
• Designs where strong colours need softening
• Pieces that will be seen with light passing through them

Over pale colours, the effect may be gentle. Over dark colours, the effect can be much stronger. Over transparent colours, Opaline can create a softer, more glowing version of the colour underneath.

This is a glass where small tests are very useful. Try one tile with a colour uncapped, and another tile with the same colour capped with 000403. You may find the capped version has a completely different feel.

Using 000403 Under Other Glass

You can also use Opaline under other glass, especially transparent colours. This gives the transparent colour a softer backing than clear would. It can be useful when you want colour and glow, but not the hard brightness of transparent colour over clear.

This can suit soft landscapes, small dishes, jewellery, and abstract work. It is less useful when you need crisp detail or clean, sharp colour. If sharpness matters, test first before committing to the final piece.

Using 000403 With Dark Colours

Dark colours can change a lot under Opaline. A dark transparent or dark opal colour capped with 000403 can become softer, smokier, or more glowing. This can be attractive, especially in small pieces, jewellery, feature strips, or design accents.

The risk is that the dark colour may lose more strength than you expected. If the dark colour is important to the design, do not assume it will stay bold under Opaline. Make a small sample first and check it in both reflected light and transmitted light.

Using 000403 Frit

Bullseye 000403 is also available as frit. Bullseye describes Opaline frit as a unique translucent style that can transmit cool tones or warm fiery effects depending on how the light hits it. Bullseye also says Opaline frit tends to strike or go opaque with extensive heatwork.

Bullseye’s tests show that Opaline frit begins to develop colour at 1400°F, which is about 760°C. It remains relatively stable through three full fuse firings to 1480°F, which is about 804°C. Bullseye recommends it for full fuse “Painting with Glass” techniques, open-faced kilncasting with minimal heatwork, frit balls, and fading colour fields.

Where Opaline frit can become less suitable is in work that gives it too much heatwork. Bullseye specifically warns against methods such as closed-mould kilncasting, where the glass is heated to high temperatures and flows from a flowerpot or crucible. Bullseye also warns that underpowered or over-insulated kilns may give the frit too much heatwork, causing it to become opaque and lose its fiery quality.

For beginner fusers, the safest way to start with 000403 frit is in simple full fuse work, small frit tests, frit balls, or soft surface effects. Once you know how it behaves in your kiln, you can use it more confidently.

Compatibility

Bullseye identifies 000403 as Bullseye Compatible.

For normal glass fusing projects, use it with other Bullseye-compatible glass. If you are using glass from a different system, do not assume it will be compatible just because the COE number looks similar. For beginner work, the safest approach is to keep the whole project within the Bullseye-compatible range unless you are deliberately testing.

Reactive Behaviour

Bullseye lists 000403 Opaline as non-reactive and neutral. That means it is not normally chosen for strong reaction-line effects. Its main value is visual: softening, warming, diffusing, and changing the appearance of other colours.

If your project depends on a reaction effect, 000403 is probably not the glass you would choose for that purpose. This does not mean every possible studio combination is guaranteed to be visually plain. Firing conditions, repeated firings, surface residue, and other materials can still affect the final result. If you are combining 000403 with unusual inclusions or materials, make a test first.

Firing Considerations for 000403

000403 does not need a special firing schedule just because it is Opaline, but the firing must suit the effect you want. If you want the milky struck appearance, a full fuse is usually the more straightforward choice. If you are tack fusing, remember that it may stay clear or only partly struck unless it has already been pre-struck.

When planning the schedule, think about the whole project: total thickness, number of layers, whether Opaline is being used as a full cap, whether air could be trapped, and whether the final piece will be fired again.

If you are unsure, use the Firing Schedule Calculator on this website as a planning aid. It will not replace testing, but it can help you think through the project before you fire it.

Opaline is especially worth testing if the project will go through more than one firing. Bullseye notes that the opacity of Opaline sheet may continue to develop in subsequent firings.

Bubbles When Using 000403 as a Cap

If you use 000403 as a full cap over another sheet of glass, think about trapped air. This is not unique to Opaline, but it matters because Opaline is often used as an overlay. If the edges seal before air has had time to escape, bubbles can form between the layers.

For Opaline specifically, Bullseye’s pre-striking advice for larger sheets also serves as a reminder that air needs a way out. Bullseye suggests fibre paper topped with ThinFire for larger pre-struck sheets, so air can escape laterally instead of gathering under the sheet as a large bubble.

If you fire a piece with 000403 and get unexpected bubbles, use the Troubleshooting Assistant on this website to work through the likely causes. With Opaline projects, pay particular attention to capping, trapped air, sheet size, shelf surface, and firing speed.

Things That Can Surprise Beginners

The biggest surprises with 000403 usually come from expecting it to behave like a normal white or a normal clear. It is neither of those things.

A few beginner surprises to watch for:

• It can look quite clear before firing.
• It may not turn milky in a low-tack fuse.
• It can change the colour underneath it.
• It can look different over light colours than over dark colours.
• Thin areas can look more yellowish.
• Thicker areas can look warmer or almost orange.
• Its opacity can increase with more heatwork.
• A pre-struck sheet may continue changing in later firings.
• Frit can lose its fiery translucent quality if it receives too much heatwork.

None of these things means the glass is faulty. They are part of how 000403 behaves.

A Simple Beginner Test Set

If you have not used Bullseye 000403 before, it is worth making a small test set. This does not need to be complicated. A few small tiles can tell you a lot.

Try making these tests:

1. 000403 by itself, fully fused

2. 000403 by itself, tack fused

3. Clear base with 000403 cap

4. Black base with 000403 cap

5. A transparent colour with a 000403 cap

6. 000403 base with a transparent colour on top

7. 000403 frit in powder, fine, medium, and coarse forms

8. One test tile fired once, and another fired twice

After firing, look at the tests flat on the bench and then hold them up to a window. Opaline can change quite a lot depending on whether you are seeing reflected light or transmitted light.

Label each test with the glass code, layer order, firing schedule, and number of firings.

When 000403 Is a Good Choice

Bullseye 000403 Opaline is a good choice when you want softness, glow, warmth, or a misty colour effect. It is especially useful when clear glass feels too hard and dense white feels too solid.

It can work well for fused glass projects such as:

• Soft landscape panels
• Cloudy or misty effects
• Water-inspired designs
• Jewellery cabochons
• Bowls and platters with a soft glow
• Abstract colour studies
• Frit balls and fading frit fields
• Overlays that soften strong colours
• Designs that will be viewed with light passing through them

It is also useful when you want to expand the colour possibilities of the Bullseye palette. Bullseye specifically describes Opaline sheet as a glass that can create new colours with distinct properties when fired over other colours.

When 000403 May Not Be the Best Choice

There are also times when 000403 may not be the right glass. If you need a simple, predictable, flat white, another white glass may be easier. If you need a clear cap that does not change the colour underneath, clear glass is a better choice.

Be careful using 000403 when:

• You need the colour underneath to stay bright and unchanged.
• You are only tack fusing and have not pre-struck it.
• You need several pieces to match exactly.
• The project will go through several firings.
• You are using frit in a process with heavy heatwork.
• You have not tested the colour combination before.

Opaline is very useful, but it is not a glass I would use blindly in an important project. A small test tile is usually worth the time.

Practical Summary

Bullseye 000403 Opaline Opalescent is a beautiful glass once you understand what it is doing. It starts out looking fairly clear, then can strike to a hazy, milky, warm light-transmitting glass after enough heatwork. It can soften colours, change overlays, create misty effects, and add a gentle glow to fused glass work.

The main thing is to respect its striking behaviour. If you want the milky Opaline look, give it enough heatwork or pre-strike it before using it in lower-temperature tack fuse projects. If you are using it as a cap, expect it to change the colour underneath. If you are using the frit, avoid too much heatwork if you want to keep its special translucent, fiery quality.

For beginners, the best advice is simple: test it first, label your samples, and look at them in a different light. Once you have seen what 000403 does in your own kiln, it becomes much easier to use it confidently.