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Properties and Manufacturing of Sapphire Crystal
Sapphire crystal is widely used in high-end mechanical watches for its extremely high hardness, excellent scratch resistance and high light transmittance. It is a kind of crystal synthesized under high pressure artificially, containing no impurities thus being clear and transparent. On the Mohs scale, sapphire has a hardness second only to diamond, reaching level 9, making it one of the hardest substances. This ensures sapphire crystal is less likely to be scratched and keeps the surface bright as new. Unlike natural sapphires, synthetic sapphire has no blue hue. It is made by pressing high-purity alumina oxide at high temperatures, then cut and polished into crystal using diamond tools. Therefore, the process is complicated and the manufacturing cost is high. This is also an important reason why many high-end watches choose to use sapphire crystal.
Composition of Synthetic Sapphire Crystal
The sapphire crystal installed on watches nowadays is actually aluminum oxide with impurities such as iron oxide and titanium oxide removed. Chemically, there is no difference between synthetic sapphire and natural crystal. No other elements are added so it is colorless and transparent without the “blue” color. The original synthetic sapphire is made from pure aluminum oxide at high temperatures, cut into discs by diamond tools, and then polished into crystal. The cost of processing tools is high, which is one of the reasons for the high cost of sapphire crystal. The technology of synthetic sapphire was invented in the 19th century and has been used in watches since the 1960s. Now top watch brands generally use sapphire crystal.
Advantages and Disadvantages of Sapphire Crystal
Sapphire crystal has significant advantages compared to regular glass and mineral glass:
- Extremely high hardness at Mohs scale level 9, only inferior to diamond, making it highly scratch resistant.
- Excellent optical properties with high light transmission.
- Good thermal stability without fatigue or cracking due to temperature changes.
However, it also has some disadvantages:
- Brittle with poor toughness, vulnerable to impacts and cracks.
- Not easy to adhere additional films like anti-glare membrane.
- Tends to attract fingerprints.
- High cost.
In daily use, avoid subjecting sapphire crystal to strong impacts. If there is dust or stains on the surface, wipe with a soft cloth. Do not use corrosive cleaning agents which may cause surface damage.
Identification Methods for Sapphire Crystal
There are some simple methods to identify whether a watch crystal is made of sapphire:
- Breath test: Breathe lightly on the surface, if the mist dissipates quickly, it indicates sapphire crystal. Generally as soon as you take your mouth away, the mist disappears when looking at the surface.
- Tapping: Gently tap the edge with fingertip. A duller sound suggests sapphire while a clearer sound indicates glass or crystal.
- Water drop test: Put a drop of water on the surface, if the drop does not spread and slowly slides down, it must be sapphire crystal.
- Touch test: Sapphire feels cooler on skin compared to other glass, giving a distinctive tactile sensation.
- Check manufacturer logo: Authentic brand name watches usually print “SAPPHIRE” or “Sapphire Crystal” on the back case to indicate sapphire crystal usage. However, counterfeits may also falsely claim this.
Feasibility of Replacing Regular Crystal with Sapphire
It is not feasible to replace a regular glass crystal with sapphire crystal on a watch. This is because the thickness and specifications differ between regular glass and sapphire crystals. Directly swapping the crystal would compromise the water resistance and integrity of the watch. Additionally, many manufacturers would refuse to service or provide warranty for watches modified in this manner.
There are several reasons why sapphire cannot be installed on watches designed for regular glass:
- The thicker sapphire piece would not fit into the existing watch case depth. This alters the original spacing and dimensions.
- Seal integrity would be impacted between the crystal, gasket and case. Resulting gaps can lead to moisture ingress and condensation issues.
- Watch accuracy may suffer from the pressure changes on the dial and movement.
- Replacing parts illegally voids factory warranty from the makers. Further repairs would be rejected.
Thus in most cases, it is not viable to replace regular glass with sapphire crystals even if sizes match. Doing so can negatively affect the workings or durability of the timepiece.
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