crystal durability
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  1. #1
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    crystal durability

    How much more durable are synthetic sapphire crystals than mineral glass crystals? I know that the synthetic sapphire crystals are more brittle than mineral glass.

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    Member Gawain's Avatar
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    Re: crystal durability

    Quote Originally Posted by Sampson777 View Post
    How much more durable are synthetic sapphire crystals than mineral glass crystals? I know that the synthetic sapphire crystals are more brittle than mineral glass.
    Mohs hardness scale is what geologists use to help identify minerals and rocks. Diamonds are 10. Quartz is 7. Glass is 6. Sapphire is between a 9 and 10..... mineral glass is between a 6 and a 7. Harder than glass but not as hard as quartz. The thing is... sapphire can shatter more easy than mineral glass and is much more expansive.

    -Gawain

    Hardness Mineral Absolute Hardness
    1 Talc (Mg3Si4O10(OH)2)
    1 2 Gypsum (CaSO4·2H2O)
    2 3 Calcite (CaCO3) 9
    4 Fluorite (CaF2) 21
    5 Apatite (Ca5(PO4)3(OH-,Cl-,F-))
    6 Orthoclase Feldspar (KAlSi3O8) 72
    7 Quartz (SiO2) 100
    8 Topaz (Al2SiO4(OH-,F-)2) 200
    9 Corundum (Al2O3) 400
    10 Diamond (C) 1500

    more:

    6.5 Iron pyrite
    6 to 7 Glass, Vitreous pure silica
    7 Quartz 7 to 7.5 Garnet
    7 to 8 Hardened steel
    8 Topaz
    9 Corundum
    10 Diamond
    >10 Ultrahard Fullerite >
    10 Aggregated diamond nanorods
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    Re: crystal durability

    i think you already know enough about sapphire vs mineral crystal

    sapphire is more scratch resistant but more brittle (for the same thickness), and more expensive

    sapphire is scratch resistant, not scratch proof. it can still be scratched, just that is a lot harder to do

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    Re: crystal durability

    More information for you:

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    'Poor Man's' Watch Forum ARCHIVE
    Understanding watch crystals ... A resource and for your edification
    Posted By: Mark S. <[email protected]>
    Date: Wednesday, 9 June 2004, at 12:52 a.m.
    Hi Guys Periodically, I see questions related to watch crystals on this forum as well as others. I took some time out (read: don't want to be doing what I need to be doing ) to gather information related to watch crystals.
    If you don't want to wade through this now, please copy, paste, and save to a notepad. I feel that it will be a great reference for you that you can refer to when wanting additional information, crystal repair, and replacement.
    This is by no means an exhaustive source of data relating to crystals, but it does offer some good information. Sources I've gotten from the internet have been credited to that source.
    Hope you like the info
    Best Regards,
    Mark
    __________________________________________________ _
    Crystals
    Three types of crystals are commonly found in watches.
    Acrylic crystal, a plastic, is inexpensive and shallow scratches can be buffed out. Acrylic crystals are a clear, durable type of plastic. Acrylic crystals are very shatter-resistant but tend to scratch easily.
    I have found that poly watch is a great polishing compound for acrylic watch crystals. You can find their product at: http://www.polywatch.de/
    Mineral crystal is comprised of several elements that are heat treated to create unusual hardness that aids in resisting scratches.
    Mineral Glass is one of the most popular and common materials used for watch crystals today. Its fine reflective characteristics, its strength and clarity, and its reasonable pricing make it a popular choice for watch manufacturers and for the watch repair industry. G-S was the originator of the term "MG" to refer to mineral glass. This term is now used throughout the watch repair industry by watchmakers, jewelers, and watch material experts.
    Sapphire crystal is the most expensive and durable, approximately three times harder than mineral crystals and 20 time harder than acrylic crystals.
    The third is synthetic sapphire, a very hard material used to make scratch-resistant crystals. Synthetic sapphire is essentially identical to the sapphire used for jewelry except that it's man-made and, when produced for watch crystals, colorless. Like natural sapphire, it rates a 9 out of a possible 10 on the Mohs hardness scale. Diamond, rated at 10, is the only naturally occurring substance that is harder. Synthetic sapphire is also used for display backs. A nonreflective coating are added on some sport styles to prevents glare.
    __________________________________________________ _
    "1.What is a watch crystal?
    A watch crystal is a transparent cover that protects the watch face. Note that, coincidently, the word "crystal" is also used to denote the tiny piece of quartz that serves as an oscillator in a quartz watch. These two types of crystals have nothing to do with each other. The latter is usually called a "quartz crystal" to prevent confusion.

    2. What are watch crystals made of?
    They can be made of any of three materials: 1- plexiglass (a clear, lightweight type of plastic), 2- ordinary glass - like that used for windows, and usually referred to in the watch business as "mineral glass" or 3- synthetic sapphire (see question 4). Some crystals are made of both mineral and sapphire glass. Seiko, for example, makes some watches with crystals made of mineral glass covered with a layer of synthetic sapphire. Seiko calls this composite material "Sapphlex".

    3. What are the advantages and disadvantages of each material?
    Plexiglass, as you would expect, is the least expensive. It is also the least likely to shatter and the most likely to become scratched. Mineral glass, even though it has been hardened by a tempering process, is more likely to break than plexiglass. But it is also more scratch-resistant than that material. Synthetic sapphire is the most expensive glass crystal material and the most scratch resistant. Because it is so hard, it is also brittle, and shatters more easily than mineral glass or plexiglass.

    4. What exactly is synthetic sapphire?
    It is a very hard, transparent material made of crystallizing aluminum oxide at very high temperatures. Chemically, synthetic sapphire is the same as the natural sapphire used in jewelry, but without the coloring agents that give the gemstone its various hues.

    When it is heated, the synthetic sapphire forms round masses that are sliced into pieces with diamond-coated saws. These disks are then ground and polished into watch crystals. (One reason sapphire crystals are relatively expensive is that the tools required to make them are costly.)
    Sapphire (whether natural or synthetic) is one of the hardest substances on earth. It measures 9 on the Mohs scale, which is a system for rating the relative hardness of various materials. (Diamond measures 10, the highest rating.) Watch crystals made of synthetic sapphire are often marketed as "scratch resistant", meaning they are very difficult - but not impossible - to scratch. Diamond can scratch them; so can man-made materials that incorporate silicon carbide, which, with a Mohs rating of between 9 and 10, is, like diamond, harder than sapphire. These materials are sometimes used to make simulated-stone surfaces for furniture or walls. The watch wearer should note that accidentally scraping a sapphire crystal against such a surface could cause a scratch.
    5. Can you tell if a crystal is made of sapphire by looking at it?
    No. Mineral glass and sapphire generally look the same. A surefire way to tell them apart (albeit an often impractical one) is with a scratch test, says Johann Jorgo, technical director at Baume & Mercier Inc. New York. A stainless steel knife or screwdriver will scratch a mineral-glass crystal but not a sapphire one.

    6. Are Scratch-resistant crystals new?
    No. Synthetic sapphire was invented in the 19th century and first used for watch crystals in the 1960s. Now really all high-end watch brands use synthetic sapphire crystals in at least some of their models.

    7. Are all scratch-resistant crystals made of synthetic sapphire?
    No. Some mineral-glass crystals are also marketed as "scratch resistant." These crystals have a hard coating that makes them less likely to get scratched.

    8. The terms "lunette", "bombé", "chevé", and "boule" are sometimes used to describe watch crystals. What do they mean?
    All are French words that refer to the shape of the crystal. "Lunette" simply means round - like a full moon (lune means "moon" in French). Bombé, chevé and boule all mean concave, or dome-shaped.

    There are other words used to describe watch-crystal shapes. A "raised" crystal is flat on top but raised up, like a birthday cake. "Shaped crystals" are any that aren't circular - rectangles, square and ovals being the most common. "Cocktail" shapes are the more exotic and extreme examples of shaped crystals. They include elongated baguette and octahedral (eight-sided) crystals.
    9. What are "anti-reflective" or "glare-resistant" crystals?
    This type of crystal has been coated on one or both sides with a substance - the same one used on anti-reflective eyeglasses - that lessens reflections and glare and makes it easier to read the watch face. Anti-reflective crystals can be made of either mineral glass or synthetic sapphire. One interesting feature of these crystals is that, viewed from the front, they are virtually invisible because they aren't reflecting any light. In some instances, the coating gives the crystal a telltale bluish tint, as it does on eyeglasses. This tint is easiest to see if the watch has a light-colored dial.

    10. How much do watch crystals cost to replace?
    Consumers can expect to pay anything from perhaps $20 to $25 for a plexiglass crystal to more than $100 for a shaped synthetic sapphire one. (At Baume & Mercier, for example, synthetic sapphire crystals range from $65 to $135.) The average cost of a round mineral crystal is about $30 to $60. An anti-reflective coating adds to the cost of any crystal. In general, the more expensive the watch, the more the consumer will have to pay to replace its crystal."

    Information found on: http://store.yahoo.com/nobelwatches/...#anchor4781980
    __________________________________________________ _
    POLISHING SCRATCHES OUT OF GLASS
    "Cerium oxide is usually the best compound for polishing glass. Glass has a hardness of slightly less than 6 in Moh's scale of hardness and will scratch most soft minerals. However, minerals with a hardness 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 cerium oxide 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 can start with a coarser 600 grit then move to the 1,200 or a 3,000 grit to remove the 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 from glass 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 used on a clean polishing pad. An electric buffer or drill works best if the item is not too delicate. Mix cerium oxide with water to create a slurry. Apply the slurry to a felt buffing pad. Harder buffing pads made out of hard felt, or leather 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 water to keep the area you are polishing damp. You can use 99.9% cerium oxcide. However, it can be a bit more expensive than the 90% Optical grade which will work fine on most glass.
    Moh's Hardness Scale
    Hardness is measured on the Moh's Scale, identified numerically hardness of by standard 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 mineral of a lower number. With a systematic approach, you can use minerals of known hardness to determine the relative hardness of any other mineral."
    Information found on: http://promagnifiers.com/forum/viewt...7457c5d3b8f78d
    __________________________________________________ _
    "Oscar Waldan on the issue of sapphire crystals:
    Sapphire crystals are composed of Corundum. In the early 1960's, they were being used for ladies watches and were faceted. These were used in very small faced ladies watches and were very expensive to produce.
    In the early to mid 1980's, sapphire crystals were beginning to be offered more for men's watches. I was one of the first to use sapphire crystals in about 1983 or so. Subsequently, Rolex began using sapphire crystals in the mid to late 1980's. On a further note, I use thick contoured sapphire crystals which cost about 3.5 times more than flat sapphire crystals. Many companies producing watches that cost thousands more than mine use flat sapphire crystals as a cost cutting method.
    Sapphire crystals are currently the best and most durable material available for watch crystals. They are synthetically made and rate 9 on the Moh's scale of hardness. The only thing harder, and capable of scratching a sapphire crystal, is a diamond which rates a 10 on the Moh's scale of hardness. Synthetic Sapphire (AL2O3), is produced from aluminum oxide. It is much less expensive than natural sapphire but is absolutely identical regarding structure, properties, and quality.
    Here are some of the technical characteristics of Sapphire Crystal:*
    Physical Properties:
    Crystalline Structure: Rhomboedral hexagonal single crystal
    Density: 3.98 gm/cm3
    Composition: AL2O3
    Thermal Properties:
    Melting Point: 2320°K 3716°F 2047°C
    Softening Point: 2070°K 3266°F 1797°C
    Mechanical Properties:
    Hardness: 9 Mohs 2500-3000 Vickers 1800-2200 Knoops
    Optical Properties:
    Transmission: Visible Light >> Excellent
    *Specs. obatined from Stettler Saphir Tech Info Library

    Our contoured sapphire crystals are high polished and burnished in to the case. That is, they are specially attached to the case by a machine under pressure and there is a gasket between the case and the crystal thereby making the case water resistant. It is similar to a hermetic seal. Case and crystal measurement tolerances are very high in order to achieve this type of seal.
    Definitions of Sapphire and Corundum:
    Sapphire:
    transparent to translucent, natural or synthetic variety of corundum (aluminum oxide, Al2O3) that has been highly prized as a gemstone since about 800 BC. Its colour is due mainly to the presence of small amounts of iron and titanium and normally ranges from a very pale blue to deep indigo, with the most valued a medium-deep cornflower blue. Colourless, gray, yellow, pale pink, orange, green, violet, and brown varieties of gem corundum also are known as sapphire; red varieties are called ruby. Much sapphire is unevenly coloured; it is also dichroic; that is, the colour of most varieties changes with the direction of view. Alexandrite sapphire appears blue in daylight and reddish or violet in artificial illumination, somewhat like true alexandrite. Careful heating and cooling under various conditions can induce permanent colour changes in sapphire (e.g., from yellow to colourless or greenish blue and from violet to pink). Other colour changes result from exposure to intense radiation. Most sapphire contains abundant microscopic inclusions; reflections from these yield a faint whitish sheen, known as silk. Tiny, regularly arranged mineral inclusions (commonly rutile) and elongate cavities are responsible for the asterism shown by star sapphire.

    Sapphire is a primary constituent of many igneous rocks, especially syenites, pegmatites, and various basic (silica-poor) types; it also occurs in schists and metamorphosed carbonate rocks. Most commercial production has come from alluvial gravels and other placer deposits, where the sapphire commonly is associated with ruby and other gem minerals. The best known sources, including some lode deposits, are in Sri Lanka, Myanmar (Burma), Thailand, Australia (Victoria, Queensland, New South Wales), India, Madagascar, Russia, South Africa, and the United States (Montana, North Carolina).
    Most transparent sapphire is faceted, generally in the brilliant style. Such gems have considerable sparkle, but they exhibit little fire because of their modest dispersion (separation of light into its component colours). Skillful cutting of unevenly coloured stones yields gems with a uniform appearance derived from only small portions of relatively deep colour. Star sapphire and other nontransparent varieties are cut en cabochon (in convex form, highly polished) rather than faceted. Despite its great hardness, some sapphire is carved or engraved, especially in the Orient.
    Synthetic sapphire has been produced commercially since 1902. Clear, sound material is manufactured in the form of carrot-shaped boules and slender rods. Much is consumed by the jewelry trade, but most synthetic material is used for the manufacture of jewel bearings, gauges, dies, phonograph-needle points, thread guides, and other specialized components; some also is used as a high-grade abrasive. Synthetic star sapphire is made with luminous stars that are more regular and distinct than those in most natural stones; the asterism is obtained through controlled exsolution of impurities.
    Top

    Corundum:
    naturally occurring aluminum oxide mineral (Al2O3) that is, after diamond, the hardest known natural substance. Its finer varieties are the gemstones sapphire and ruby, and its mixtures with iron oxides and other minerals are called emery.

    Corundum in its pure state is colourless, but the presence of small amounts of impurities can impart a broad range of hues to the mineral. Ruby owes its red colour to chromium, sapphire its blue shades to the presence of iron and titanium; most corundum contains nearly 1 percent iron oxide. The mineral readily weathers to other aluminous minerals, margarite, zoisite, sillimanite, and kyanite.
    Corundum crystallizes in the hexagonal system, forming pyramidal or rounded barrel shapes. It is widespread in nature, being found in igneous, metamorphic, and sedimentary rocks. Large deposits are rare, however. Some of the richest deposits occur in India, Russia, Zimbabwe, and South Africa.
    In addition to its use as a precious gem, corundum finds some use as an abrasive, owing to the extreme hardness of the material (9 on the Mohs hardness scale). It is used for grinding optical glass and for polishing metals and has also been made into sandpapers and grinding wheels. Because of its high melting point (2,040°FC, or 3,700°FF), it has also been used in refractories.
    In most industrial applications corundum has been replaced by synthetic materials such as alumina, an aluminum oxide made from bauxite. Artificial corundum may be produced as a specialty product, as for gem use, by slow accretion and controlled growth on a boule in an oxyhydrogen flame. This procedure is known as the Verneuil process."
    Info found on: http://www.waldaninternational.com/sapphire.shtml
    __________________________________________________ _
    CVD Technologies
    "Mainland China – Shenzhen Leidi Science & Technology Co. Ltd has developed a diamond membrane for use as a low-cost option to sapphire crystals.
    Leidi used CVD technology to apply a layer of diamond membrane onto a substrate. The finished crystal has properties similar to that of sapphire, but at a lower cost. Diamond crystals are hard and resist abrasion.
    Diamond membrane has been used for various purposes since the 1980s. Diamond purity is usually between 60 percent and 90 percent, resulting in translucent membranes. But Leidi sales representative Hua Dianbo claims the company can produce diamond membranes with a purity of up to 99 percent, yielding transparent films. Leidi's diamond membranes can be applied to metal, glass and plastic substrates.
    In addition to watch crystals, the diamond membrane can also be used as a harder coating alternative for metal watchbands. Leidi carries the patents for the diamond membrane watch crystals and bands, with respective patent numbers ZL97114300.5 and 01242407.2."
    Information found on:http://www.globalsources.com/am/arti...ion=GetArticle
    __________________________________________________ _
    "How is a watch crystal replaced? First, let us consider wrist watches and pocket watches separately.
    1- Wrist watches. Wrist watch crystals come in several materials: primarily they are plastic, glass, and synthetic sapphire. The sapphire crystals are very hard, difficult to produce, and expensive. They are used in the more expensive watches such as Rolex, Breitling, etc. Wrist watch crystals may be round or fancy shaped (anything besides round). The round crystals may have a high or low profile. Most quartz watches have a mineral glass crystal with a plastic gasket ring. The plastic crystals may or may not have an inner ring of metal (tension ring) to support them. Crystals for diver's watch crystals, which must be very strong often have the tension ring. There are special crystal lifts made to compress the round high dome plastic crystals to remove and insert them. Replacing a fancy (non-round crystal) on a vintage watch may take a lot of time. Genuine crystals made by the manufacturer are often not available. Thus, the glass or plastic crystal is a generic replacement, which may not fit perfectly. Sometimes it is too large and must be filed or ground down, and then the edges must be repolished. These days, most of the elite Swiss watch manufacturers will not supply genuine case parts such as crystals to watchmakers. They want you to send your watch back "to the factory repair facility" for all repair work. That is why I specialize in vintage and antique watches, I don't want the headache of trying to get parts from the modern watch manufacturers.
    2- pocket watch xtals. Pocket watch crystals are almost always round with a few exceptions. The crystals were originally glass in the old watches. The oldest crystals made in the 1600's through the early 1800's were hand blown, and are sometimes called "bull's eye" crystals due to the flat center portion where the blowpipe's mark is ground out. More modern pocket watch glass crystals can be heavy or thin, nearly flat or domed. Glass crystals must fit almost perfectly, which means that they have to be measured very precisely to replace them. If an exact fit is not possible, a near-fit can be held in place with an adhesive such as epoxy or a resin that sets under ultraviolet light. The diameters of more modern glass crystals; i.e, ones made in the last 50 or 60 years are often measured in millimeters. Hunting case crystals are difficult to fit as they are thin, and the cover must not hit the crystal and the hands must clear from below. Thus, both the diameter, the height, and the thickness of the crystal must be matched. One big problem is that quite often the "popular size" glass crystals have all been used up, especially for the 16 size American watches. Although plastic crystals generally work well in pocket watches, some collectors want the glass crystal when possible. There are special tools to compress the plastic crystals in order to insert them into the case. The older glass crystal diameters were measured in lignes. This is a French system of measurement that is commonly used by watchmakers and is equal to 2.25583 mm."
    Information found on: http://members.tripod.com/~watchdoc/faq.htm#xtal
    __________________________________________________ _
    Messages In This Thread
    Understanding watch crystals ... A resource and for your edification (views: 4760)
    Mark S. -- Wednesday, 9 June 2004, at 12:52 a.m.
    [ View Thread ] [ Return to Index ] [ Read Prev Msg ] [ Read Next Msg ]
    'Poor Man's' Watch Forum ARCHIVE is maintained by PMWF Admin team with WebBBS 5.11.
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    Orient: Black Mako (FEM65001BW)
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    Seiko: SKX781 Orange Monster, SNK807 Seiko 5, SUN017P1, SNP105 (incoming)
    Tissot: PRC200
    Other: Stuhrling Skeleton (1070.33161)

    "Don't wait for the perfect moment, take this moment and make it perfect."
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  6. #5
    Member tribe125's Avatar
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    Re: crystal durability

    That ought to cover it.
    I used to list my watches here until I realised it ruined people's Google searches...

  7. #6
    Member casiophile's Avatar
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    Re: crystal durability

    Wow. Is there gonna be a quiz? :)

  8. #7
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    Re: crystal durability

    Thanks for all the info. Guess that's a complete answer.

  9. #8
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    Re: crystal durability

    Good info, thanks!

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