Watch crystal considerations

[vavavroom]

The five people on the board who enjoy watches may know this already but I got bored.

Different materials are used for watch crystals. In most cases, a watch crystal can be replaced with an identical one or with an upgraded one. For example, you can go from a mineral crystal to a sapphire crystal. Sometimes, a watch may be available with different crystal choices.

Quartz glass: optically very clear, low expansion, chemically resistant, not very scratch resistant, fragile, can shatter into shards.

Celluloid, a bio-plastic, a carry-over from late 19th-century pocket watches: degrades over time and changes from clear to yellow or blue/green, does not resist heat well, is highly flammable, develops crazing with age

Acrylic beginning in the 1920s: shatter-resistant but very soft and scratches easily. Light scratches can be buffed out with plastic polish like Polywatch. Chemically not very resistant. Goes by names like Hexalite (Omega). Replacement is cheap.

synthetic sapphire since the 1930s: Most scratch-resistant but less hatter-resistant than mineral glass, although modern sapphire crystals do not shatter easily. Sapphire is naturally less prone to reflections than mineral glass. More costly than mineral glass but not prohibitively so. Scratches can’t be removed by the layman.

Mineral glass and high-strength mineral glass since the 1970s: this tempered glass is a good compromise between scratch-resistance and shatter-resistance. Goes by names like Hardlex (Seiko). You may be able to remove hairline scratches with glass polish. Replacement is cheap.

Resin, probably since the '80s: scratches easily but shatter-resistant. Found mostly in cheap watches that have the case and crystal injection molded in one piece. Think cheap Casio watches with non-replaceable crystals.

Sapphire-coated mineral glass. I'm not sure when these crystals first appeared but I'm thinking 1970s: has the shatter resistance of mineral glass with increased scratch resistance. Goes by names like Sapphlex and Flame Fusion. Layering affects optical clarity negatively. The coating can delaminate and is not really superior to hardened mineral glass.

In short:

Mineral glass offers a good compromise between price, scratch, and shatter resistance.
Sapphire crystals are a premium option and from what I can tell shatter resistance is now close to that of mineral glass
Acrylic is best when a very particular look is required that only acrylic can give. The shatter resistance is offset by its non-existent scratch resistance but YMMV.
Resin is fine for beater watches

Crystal coatings:

Anti-reflective coatings are either clear or impart a color, often blue. Hydro and oleophobic coatings keep the crystal free from smudges and water/sweat droplets until the coating wears off over the years

Coatings are either on the top of the crystal, on the underside, or on both sides. On both sides offers the least reflections but the outside coating will eventually get damaged which looks ugly. The coating can be easily removed with a glass polishing compound. I prefer an anti-reflective coating on the underside of the crystal as it will never wear off.


Acrylic/resin watch crystal polishing compound

 

[CharBaby]

Okay, I jump in here!
Everything you posted is what I have learned as well about watch crystals.
Nice to hear from members who like watches as I do.
There are crystals that I am fond of more than others.

For example, I like "Flame-Fusion" over "Hardlex". I have scratched Hardlex easily but have been able to polish out the scratch with rouge and a buffer. I do like sapphire best of all but I am not that particular whether or not it has an anti-reflective coating. Although if it does have an anti-reflective coating, I'd prefer if it were under the crystal as it won't scratch as you mention. I have heard of anti-reflective coatings being on top of the crystal but why do watch makers do this?

I have shattered a sapphire crystal(sadly) but it was easy to replace and was not expensive IMO.
I don't particularly like acrylic crystals but they do indeed polish out light scratches very easily with toothpaste or as you are showing, polyWatch. Or just replace the crystal. I have overheated the acrylic crystals by using a buffer and this is a NO-NO! You must go very slow.

Thanks for posting this subject,

 

[Jetronic]

I thought this was going to be about quartz crystals lol

 

[Lubener]

Whatever resists scratches the best.

 

[MParr]

Synthetic Saffire

 

[Kira]

Many thanks for the informative post.

From my perspective, a watch crystal need only meet two requirements:
a) Something with reasonable scratch resistance.
b) Standardization of shape (no oddball shape).
Specifically: I had a watch crystal which, when measured in cross section, must've been extra thin.
Replacement crystals had a 'beefiness' around the outer edge which touched the end of the hour hand.

 

[Hermann]

The only crystal I have damaged was a Flame Fusion on an Invicta 1953. Barely noticeable if your looking for it, so I haven't done anything to it.

 

[Al]

Not a watch guy, but interesting conversation!

 

[vavavroom]

I thought this was going to be about quartz crystals lol

I don't want to disappoint you. No healing/magical quartz here!

Let's look at the history of clocks and watches. Nobody knew about Einstein and his relatively zany ideas when other clever men first built clocks. For our simple purpose of making it to work on time, we can safely disregard such newfangled magical nonsense like time dilation and whether time even exists.

The Egyptians began using water clocks and sundials around 3,000 years ago. Fast forward to 1300 and the invention of the first oscillating escapement, the crown wheel escapement. Oscillating escapements of newer designs are used in modern mechanical watches and oscillation is the principle behind electric and quartz watches. The oscillation frequency is divided my mechanical or electronic means into 1Hz or a fraction thereof. For example, a modern fast-beater mechanical movement oscillates at >4 Hz or 8 ticks per second or 28,800 vibrations per hour. A higher beat rate, presuming consistency, yields greater accuracy potential and finer resolution for chronographs.

The idea of using a tuning fork as an oscillator to build a highly accurate clock was first realized by Niaudet in 1866. This was a purely mechanical device and a modified version with spinning mirrors was used by Michaelson to measure the speed of light in 1882. Amazing!

Quartz exhibits a piezoelectric effect:* induce mechanical stress and quartz will produce an electric field/electric charge, apply a current and the quartz will vibrate at its natural frequency of of 33KHz. A tuning fork made out of quartz was devised. Quartz tuning forks are made by cutting a blank from quartz and lapping/laser-trimming the quartz to increase it's frequency to the desired 37,768KHz. The first quartz clock was built in 1928 at Bell Labs. It was a little large and required a room for storage.

Miniaturization of electronic components allowed making electric tuning for watches and clocks in the 1960s. See Bulova Accutron and the clocks NASA used in the space program. Eventually, miniaturization allowed for quartz wristwatches. The Seiko Astron was the first quartz wristwatch in 1969. I have seen quartz crystal cases that were over 10mm long. Modern quartz crystals are usually a few mm short but SMD versions may be just over 1mm tiny.




*The piezoelectric effect is commonly used in electronic lighters that contain a piezoelectric electro-mechanical igniter module that consists of a spoingloaded hammer pounding a quartz crystal to produce and electric spark for ignition.

 

[vavavroom]

The only crystal I have damaged was a Flame Fusion on an Invicta 1953. Barely noticeable if your looking for it, so I haven't done anything to it.

Did the coating delaminate or chip off?

 

[vavavroom]

Many thanks for the informative post.

From my perspective, a watch crystal need only meet two requirements:
a) Something with reasonable scratch resistance.

In that case, sapphire is the best choice. A sapphire crystal may cost as little as $30. If you have a very common watch, for example, a Seiko SKX, upgrading to a sapphire crystal is no problem. I even have a cheap Casio that sports now a sapphire crystal although the watch came with a resin crystal. The watch cost $25, the DLC-coated stainless steel case with sapphire crystal I bought was over $100.

b) Standardization of shape (no oddball shape).

That would limit design options. Crystals not only have different circumferential shapes but they may be flat, domed, double domed, with or without beveled edges. They may be thin for dress watches but over 5mm thick for dive watches. And so on.

Specifically: I had a watch crystal which, when measured in cross section, must've been extra thin.
Replacement crystals had a 'beefiness' around the outer edge which touched the end of the hour hand.

You just have to find the correct size crystal.

The difference between mineral glass and sapphire becomes obvious when you have two of the same watches side by side. The one with the sapphire crystal exhibits a clarity of the dial and hands the mineral crystal can't match.

 

[Hermann]

Did the coating delaminate or chip off?

Not that I noticed, it's not on the edge. It is probably just from a hard whack from a doorknob. Not likely a direct head-on hit, mostly likely at an angle. A small whiteish area that's longer than wide, barely noticeable when looking for it.

 

[vavavroom]

Okay, I jump in here!
Everything you posted is what I have learned as well about watch crystals.
Nice to hear from members who like watches as I do.
There are crystals that I am fond of more than others.

For example, I like "Flame-Fusion" over "Hardlex". I have scratched Hardlex easily but have been able to polish out the scratch with rouge and a buffer. I do like sapphire best of all but I am not that particular whether or not it has an anti-reflective coating. Although if it does have an anti-reflective coating, I'd prefer if it were under the crystal as it won't scratch as you mention. I have heard of anti-reflective coatings being on top of the crystal but why do watch makers do this?

I have shattered a sapphire crystal(sadly) but it was easy to replace and was not expensive IMO.

I have never managed to crack a sapphire crystal but I have chipped one.

I don't particularly like acrylic crystals but they do indeed polish out light scratches very easily with toothpaste or as you are showing, polyWatch. Or just replace the crystal. I have overheated the acrylic crystals by using a buffer and this is a NO-NO! You must go very slow.

Thanks for posting this subject,

Using a very large mop/buffer and low RPM would be safer versus using a smaller mop/buffer and high RPM. Replacement is the best option if you don't have to ship the watch cross-country.

 

[Astro14]

Breitling coats the sapphire crystal both inside and out with anti-reflective coatings to reduce reflections in the cockpit where the sun can be intense. The sapphire is hard to scratch, but the coating is not.

 

[vavavroom]

Here's an example of sapphire vs resin crystal. The clarity of the display is much greater with the sapphire crystal and the resin crystal exhibits more reflections. Note where it says MODE on the dial. There is reflection of MODE to the left of the lettering on the watch with the resin crystal. You can also tell that the sapphire crystal is very flat while the resin crystal is a bit warped causing a gradient reflection on the surface.

The watch with the resin crystal on the right has a non-replaceable crystal because the case and crystal are injection-molded in one unit. However, a DLC* coating stainless steel aftermarket case with sapphire crystal is available. The black resin watch looks smaller because it's a few millimeters farther from the camera.

*Diamond-Like Carbon coating - offers great surface hardness and scratch resistance at up to 9,000HV or 9,000 kg-force/square mm.

 

[Carmudgeon]

The only crystal I've ever managed to damage was on an 1980s TAG Heuer F1, which I see have now finally taken their turn as a retro revival/fad.

It's on its second movement (under warranty), second resin strap, and second crystal. Needs a battery, but still works.

Recently gave the Vostok (gifted by a friend who visited Russia circa the collapse of the SU) a new strap, and buffed up its acrylic crystal.

When I see the wrist monstrosities resulting from the offshoot of the "cell phone protection industrial complex," applied to smart watches, I wonder if some people have lost the plot.

 

[GMBoy]

I love watches too! I thought, at first, you were referring to the quartz in the time keeping mechanism lol.