Do you know how to evaluate the transparency of a glass cup

Many buyers think that thickness determines transparency. The difference in light transmittance between 0.5mm and 3mm glass may be less than 1% 

 The following aspects affect transparency.

1. Composition and transparency basis of soda-lime glass.

Soda-lime glass is a common material for daily glasses. Its main components are:

SiO₂ (70-75%): provides glass network structure, high purity reduces light scattering.

a₂O (12-16%): reduces the melting point, but excessive amount increases light absorption.

CaO (8-12%): enhances chemical stability, but may introduce trace impurities (such as iron ions).

Key challenges:

Iron (Fe³⁺) impurities in the raw materials, even as low as 0.1%, will cause the glass to turn green (absorb blue light) and significantly reduce transparency.

2. Process for improving the transparency of soda-lime glass.

(1) Raw material control

High-purity quartz sand: Fe₂O₃ content must be <0.01% (such as using low-iron sand from Australia or Malaysia).

Addition of decolorizing agent:

Selenium (Se) and cobalt (Co): complementary colors neutralize the green color of iron ions (such as Se produces a light pink hue).

Cerium oxide (CeO₂): oxidizes Fe²⁺ to Fe³⁺ (light yellow is more acceptable).

(2) Melting and clarification process

High-temperature melting (1500-1600℃):

Fully decompose impurities and release bubbles with clarifiers (such as sodium sulfate, Na₂SO₄).

Reducing atmosphere control:

Introducing natural gas (CH₄) into the melting furnace to reduce Fe³⁺ (yellow) to Fe²⁺ (green).

(3) Molding and annealing

Blow molding (mechanically molded cup body):

The mold surface is chrome-plated and polished to ensure a smooth glass surface (roughness <0.1μm).

Precision annealing:

Slowly cool in a 550℃ annealing furnace to eliminate internal stress (avoid stress birefringence causing haze)

3.  FAQ for the buyers of glassware and tableware 

Q: Why does my glass look less transparent than the store sample?

A: It may be due to:

High iron content in the raw material (cheap glass);

Insufficient annealing causes internal stress to scatter light;

Micro scratches after repeated surface cleaning (accumulated haze).

4. How to distinguish between crystal glass and a soda lime glass cup?

Visual light transmission: Point the cup at the text, and the edge of the crystal glass text is clearer, while the ordinary glass is slightly blurred.

Edge color: The edge of soda-lime glass is green, while crystal glass is colorless or very light blue-gray.

Knocking test: The sound of crystal glass is like a bell, with a long reverberation; the sound of soda-lime glass is short.

5. Suggestions to the buyers for the glass cup. 

Choose crystal glass:

High-end wine glasses (such as red wine glass goblets), handicrafts, and decorations that require optical effects.

Pursue the artistic sense of "crystal clearness" and light refraction. The crystal wine glass is most popular among our buyers come from Europe, in gift orders.

Choose soda-lime glass:

Daily use of glass water cups, storage jars, and other economical glass tableware. These kinds of buyers focus on practicality and the cost of the glassware, not the appearance.

Such as a promotional project, empty glass storage  jars for instant coffee powder, the wholesaler or the importer prefers an IS machine-made glass storage jar for 50g, 100g, or 200g of instant coffee powder 

However, both the crystal glass and soda-lime glass can meet the food-grade standard, but the color of the glass cup looks different.

As a leading professional glassware factory in China, and has become an international tableware supply chain since 2011.

Garbo Glass has more than 300 employees in the factory and an international business team of nearly 100 people in the Guangzhou headquarters.

Our sales team will present the most suitable proposal based on your needs and market.