How to test for Sapphire Crystal?

[miket900]

Hi All,

I wonder if you can help? I am intrigued by the methods used to test if a crystal is Sapphire or not. I have found posts about tapping the crystal, placing water drops, scratching with a screwdriver and scratching with glass. My question is: If you place a drop of water on the surface, what exactly are you looking for to confirm it is Sapphire? What are you looking for with a tap?

I'm not overly keen on the scratching technique as I have read stories of 'explosions'!!

I'd appreciate any advice you may be able to put my way.

Thanks,

Mike.

[Agrippa]

If you're using the tapping method, a sapphire will emit a low frequency sound, while tapping a mineral crystal will produce sound of a higher frequency. The reason is quite simply that a sapphire has far greater density than a mineral crystal and hence is less excitable (i.e. it is much harder to induce vibrations in it, which is what produces sound). However, the sound is also dependent on the size and thickness of the crystal, so the difference will be less pronounced if the sapphire crystal is thin while the mineral crystal is thick and vice versa.

[redwatch]

Doing a search for "Crystal Test" gave me 19 pages of results. Out of those 19 pages, I found this link on page 12:

Definitive Sapphire Crystal Test?

That link led me to this one:

Two Electronic Tools To Distinguish Sapphire From Glass

[nakortheblue]

See if you can pick up a diamond tester. It tests the conductivity of heat. Sapphire and diamond will conduct heat better than glass.

[P4GTR]

See if you can pick up a diamond tester. It tests the conductivity of heat. Sapphire and diamond will conduct heat better than glass.

I wonder if those jeopardize AR coatings in any way. Direct contact and heat sound like a no-no. Just speculating...

[miket900]

So what is the relevance of putting a drop of water on the crystal, what does that do?

[offshore]

On our website, www.watchbitz.com.au, we publish a number of documents which try to assist repairers.

This is one.-

So, you need to polish a watch crystal?

The first thing we see when we look at our watch is the glass which protects the dial and hands, its crystal.

A scratched or marked crystal is visually distressing, and devalues your treasured timepiece.

Instead of replacing a scratched crystal, which can become expensive, you can look at polishing it, to return it to its former glory.

First, however, you need to know what material your crystal ismade of.

There are basically three materials used to make crystals, Acrylicor plexiglass, Mineral glass, and Synthetic sapphire. It can be sometimes difficult to know exactly what material yours is.

(There are some other composite crystals which are a mixture of those listed above. We will deal with them separately)

Firstly Acrylic. If you tap an acrylic crystal with your fingernail you will hear a "tink" sound, (as against a distinct deeper "clunk" with the two glass crystals.) Acrylic is the softest of the three, and tends to markand scratch the easiest. Minor marks on an acrylic, can be removed, bypolishing with toothpaste. Deeper scratches may require an initial polish withBrasso, followed by a final polish with toothpaste. We also supply an excellent product called Polywatch, which in addition to having the polishing power of toothpaste, also has a product in solution, which melts a layer of the acrylic surface, and flows into the deeper scratches, hastening the polishing process.On rare occasions, an initial cut with Brasso may be required before using Polywatch. Exceptionally deep gouges may even require the application of a fine(800 or 1000 grit) wet and dry abrasive sandpaper to remove them, before moving through to the next polishing stages as previously detailed.

Next Mineral glass Vs Sapphire. To confirm the material used in the construction of a glass crystal, the surefire way is to test it with adiamond tester.

Many watch collectors and repairers have also developed a sense of the different sounds made when tapping sapphire and mineral glass crystals with a finger nail, although this is an acquired skill.

Also mineral glass feels cool to the touch, whist acrylic andsapphire glass, have a neutral or room temperature feel.

Next is the colour test. Look at the edge of the crystal (not straight down but from the side) A lamp or other light on the opposite side of the watch will help. Acrylic crystals in the main are clear or white. A sapphire crystal will have a clear or maybe a pinkish or white hue; a mineral crystal will almost always have a greenish hue. Not foolproof.

Last, the water test -place large drops of water on the crystal, then tilt the watch and watch the water slide off. On the sapphire crystal, water will easily slide off - leaving small well-formed round globules of water on the surface. On the mineral crystal - most of the water will "stick" to the crystal and the drops of water will spread across the surface of the crystal to form a non-uniform "puddle"instead of sliding off. Make sure the crystal is clean, and completely free of wax and oil, as this can confuse this test.

Having reasonably confidently defined a mineral glass from a sapphire, a means to polish is needed.

Any number of mild abrasives can do this, ranging from 'Flitz'which can be obtained from gunshops, to a fine stainless steel polish, carwindscreen polishes, and cerium oxide.

Cerium oxide is usually the best compound for polishing mineral glass. Glass has a hardness of slightly less than 6 in Moh'sscale of hardness and will scratch most soft minerals. However, minerals with ahardness of 6 or more will easily scratch a piece of glass. Before you begin polishing a piece of glass you must first determine how deep the scratches are in the material. First run your fingernail across the item to be polished. If you can feel the scratch, it may be too deep to be polished out with ceriumoxide and may require more aggressive grinding.

The grit size used to grind out a scratch always depends on the depth of the scratch to be polished. Starting with a fine 1,200 grit size would be best. However, this may be slow. If this is the case you canstart with a coarser 600 grit then move to the 1,200 or a 3,000 grit to removethe scratches from the coarser grit size. You can now proceed to polish the glass with cerium oxide. Another great way to grind out the scratches fromglass is to use a diamond powder with oil as a lubricant.

After each grinding, clean the area you have ground with alcohol before going to the next finer grit size. Each grit should be usedon a clean polishing pad. An electric buffer or drill works best if the item isnot too delicate. Mix cerium oxide with water to create a slurry. Apply theslurry to a felt buffing pad. Harder buffing pads made out of hard felt, orleather work best.

The polishing action will go faster when the cerium oxide is damp, but not dry or too runny. You can use a spray bottle with waterto keep the area you are polishing damp. You can use 99.9% cerium oxide.However, it can be a bit more expensive than the 90% Optical grade which willwork fine on most glass.

Moh's Hardness Scale

Hardness is measured on the Moh's Scale, identified numerically hardness of bystandard minerals, from 1 (softest) to 10 (hardest):

1. Talc

2. Gypsum

3. Calcite

4. Fluorite

5. Apatite

6 .Orthoclase

7. Quartz

8. Topaz

9. Corundum

10. Diamond

A mineral of a given hardness will scratch other minerals of a lower number. With a systematic approach, you can use minerals of known hardness to determine the relative hardness of any other mineral.

Finally we look at Sapphire glass crystals, by far thehardest to scratch, but also the most difficult to polish.

Sapphire glass is a synthetic material with a very high hardness, (around 9 on Moh's scale, as above.)

To polish it, we need a product harder than it, so here a diamond polishing paste is used. We have put together a kit of 2 grades of diamond paste, for this job. A 3.5 micron paste for the initial cut to remove deep scratches and a 0.5 micron paste, for the final polish.

You may need to polish a sapphire crystal for sometime, using the 3.5 micron (red)polish. (Up to 20-30 minutes to achieve a goodresult) Always check every few minutes, as to how the polishing is progressing,and once the crystal is free of scratches, apply a final polish using the 0.5 micron(rose) polish.

Finally we had mentioned some other composite crystals. Seiko put out a crystal they call Saphlex.

It is a composite crystal however the outer layer is sapphire, so polishing requires the same technique. They also have a hardened mineral glass crystal called Hardlex. It is slightly harder than mineral glass,but again requires the same techniques as for standard mineral glass, maybejust a little more perseverance!

One final point to consider- some crystals (mainlysapphire) are treated with an AR (anti- reflective) coating. Attempting to polish a crystal with AR, will damage this coating, if it exists.

The information within has been gathered from many resources on the'net and sounds correct to us.

Also, if you are using a diamond tester, you are measuring, not applying heat. A diamond tester will not damage AR.

Offshore

[Guest avitt]

Coincidentally, I just received a new diamond tester from Watchbitz a couple of days ago. I've used it to test every watch that I own (about half of which have double AR), and its infallable

[offshore]

Glad it arrived avitt, and that you are satisfied with the unit.

We did test quite a few of these before deciding on that particular one.

It isn't the cheapest going around, but it does give an accurate result, and has some longevity.

Offshore

[Guest avitt]

I'm pleased with the entire order, Offshore

[freddy333]

When viewed from the side, mineral glass (like most glass) has a green tint. Sapphire is white or clear, depending on the quality of the material & finish. As a double-check, I use the finger test (performed at normal room temperature) - mineral glass always feels cool to the touch, while sapphire feels like room temperature. Plastic/Plexiglas/acrylic just feels like plastic.

[miket900]

Excellent info, thanks for all the help!