Why Do Some Glass Candle Jars Have Seam Lines?

Seam lines on glass candle jars are a normal result of the manufacturing process, not necessarily a defect. They are formed during molding and are common in mass-produced glass containers.

How Are Seam Lines Formed?

Most glass candle jars are made by:

• Press molding (pressed glass), or

• Blow molding (machine-blown glass)

Both methods use metal molds that close together to shape molten glass.

When:

1. Molten glass is poured into the mold

2. The mold closes and shapes the glass

3. The glass cools and solidifies

The joints where the mold pieces meet leave a thin line on the surface.

👉 This line is called a mold seam line.

It is similar to the molding line seen on plastic or injection-molded products.

Differences by Production Method

1. Pressed Glass (most common & cost-effective)

Features:

• Visible seam lines

• Straight vertical lines along the side

• Slightly raised in some cases

• Suitable for mass production

Commonly used for:

• Candle jars

• Home fragrance containers

• Everyday glassware

Seam lines are normal for this type.

2. Machine-Blown Glass

Features:

• Finer and less noticeable seams

• Smoother surface

• Better overall finish

Often used for:

• Mid-range candle jars

• Premium-looking containers

3. Hand-Blown Glass (high-end)

Features:

• Usually no seam lines

• More natural and artistic appearance

• Slight thickness variation

• Much higher cost (3–5× more expensive)

Used for:

• Luxury candles

• Designer or artisan products

• Custom or boutique brands

Are Seam Lines a Quality Problem?

Normal (not a defect)

If the seam is:

• Smooth

• Not sharp

• Not cracked

• Not affecting strength or safety

It is simply a manufacturing mark and does not impact use.

Potential defects (should be avoided)

If you notice:

• Sharp or rough edges

• Cracks along the seam

• Air bubbles concentrated on the seam

• Structural weakness

These may indicate poor production quality.

Why Don't Manufacturers Remove the Seam?

Mainly because:

1. Cost

Grinding or polishing seams:

• Increases labor

• Reduces production efficiency

• Raises product cost by 30–50% or more

For standard candle jars, this is usually unnecessary.

2. Structure

In some cases, seams actually add slight structural support, so removing them completely is not always ideal.

How to Reduce or Hide Seam Lines?

Manufacturers may use:

• Fire polishing (heat smoothing)

• Frosted or matte finishes

• Coating or painting

• Electroplating or spraying

• Switching to hand-blown production

These methods improve appearance but increase cost.

Quick Summary

Seam lines are a natural result of mold-based glass production, not a defect.

Only rough, sharp, or cracked seams should be considered quality issues.

For most candle jars, seam lines are completely normal and safe.