Abrasives for the "glass scratcher" (italian "Grattavetro")

Features employment – Grits abrasives to achieve Findability – granulometric Sequences – mirrors for telescopes.


As already mentioned in other articles on the making of mirrors for telescopes, amateur   current processing of such an object requires a raw glass disk that will be the mirror, be rubbed with her Center against the the edge of a second glass disc acting as a tool,  with interposition of abrasive powder and wet, that will bring the contact areas to be consumed, with the mirror that will take on a concave surface and spherical,   flared then in parabolic shape in the final stages of processing; While the surface of the tool changes complementary convex.


Those powders are then fellow travelers of the "student", until completion of his work. They are made of very hard materials with sharp and cuttigs edges, that for pressure contact shatter and grind the glass in crumbs with average size comparable to the size of their grain , or their "grit" (both terms are equivalent to the anglo american term “Grit” -).


The abrasives manufacturer must make available to buyer material safety data sheets and use of all materials and products sold. But potentially greater danger of whole machining, remains to inhale the glass in the form of silica at the molecular level.

But from the technical point of view, all stages of processing must take place “wet”. It should be known that if this happens in reality,   is averted any chance of inhalation of glass, and as a result the processing, as for centuries is made, hazard problems would not arise.


Each abrasive is characterized by a number of grit, that expresses the number of stitches per square inch   of sieve used to obtain its precise separation by particles of different size. Although today actually this is only for coarse grits, ranging from 16 to the 220.

Beyond the grain 220, that is, from 240 to the 3000, many companies no longer use sieves, but operate the separation to "Elutriation", i.e. sedimentation in water of powders. Where larger grain size powders, settle before the other. For example the powder pitch diameter 100 Micron sediments in 1 minute, While the powder with medium grain 16 Micron sediments in an hour: Then the first will name (For example,) of emery 1 ', While the second Emery 60 '.

NOTE: Modern technology makes us forget that, also in not so ancient times, intelligence overcome completely to it. And the lack of commercial availability of an abrasive refined in wel refined and guaranteed particle size , It was filled from “prehistoric” sedimentation process, that allowed (and still allows) Soak some amount of abrasive grind, and after waiting the necessary time (possibly throwing away coarse deposits obtained until that moment), to obtain from then on, a perfectly fine grain size sedimented (without using machines, How could they already get the cavemen). END OF NOTE

The habit of professional jargon in the industries is very hard to change, So what, although it is perhaps more useful and easy to understand by calling an abrasive with its average grain diameter, BUT you are still using different encodings that needs necessary tables of equivalence between various grits and its true average sizes, -.


Abrasive grit size table with equivalencies in microns

Abrasive types used in the manufacture of mirrors are:

  •   silicon carbide called universally Carborundum;
  •   cast aluminum oxide called aluminium oxide (but it is also called Garnet, Emery Emery or French or  );
  • industrial Diamond  ;
  •   Zirconium Oxide;
  •   Cerium oxide;
  • Iron oxide  .

Making telescope mirrors means to use abrasive powder in decreasing grain size, from the initial stages to final processing.

The work usually starts with the laborious hogging or excavating the curve, that is done by using   very coarse grits, such as the 60 for medium to large mirrors (medium grain 268 microns, then about 2.5 tenths of mm); or grit 80 for small and medium   mirrors (medium grain 192 microns, ( that means 2 scarce tenths of a mm) so that excavation proceed with greater speed and efficiency in the removal of the large amount of glass.

Once is reached the depth named "arrow" in the shell of the desired sphere abraded in that way, further processing with different sessions, in each of which uses a progressively abrasive grain with about half size compared to the previous, up to refine the surface shiny again getting it perfectly.

The change of grain takes place as soon as,   careful examination of the surface in production, Maybe using means of magnification, the current Abrasives have erased any trace of rough edges left by the previous grain.


SILICON CARBIDE: It's a Ceramic material “extraterrestrial” with hardness 9 Mohs scale.

The Mohs hardness scale start goig back from hardness 10 Diamond, that is the highest absolute hardness of any material.

A hardness 9 It is therefore only second to Diamond.

The definition of extraterrestrial material is due the fact that we now know silicon carbide being very common in interstellar space and in meteorites, Why product nearby red giant stars that synthesize carbon   in large quantities with nucleosynthetic process of merging three helium atoms.

"On Earth" in its natural state is very rare, but it is produced artificially by 1893 (Discoverer the American company "Carborundum") using sand and coke coal in electric furnaces with graphite resistance to temperatures of 2500° c, in the vicinity of which form Pure crystalline silicon carbide Green, that moving away from the electrodes crystallizes in a less pure and less abrasive changing color in black. It is sold wholesale in bags 25 kg with particle sizes from 24 to the 1500 While its retail sale in Italy is less easy to find.

  • USE: It is the most widely used abrasive used (also personally) in all phases of the spherical surface of mirrors: From the initial pressing, with the grits much coarser than the larger the mirror to realize, at the refinement “Satined surface” with the grit 800 (I've never used the grain 1000 and I never had any problems of roughness and subsequent polishing).

FUSED ALUMINIUM OXIDE, synonym of corundum (or Emery, -, or Garnet): It 'a ceramic material with hardness 8 Mohs scale. Its grits are normally finest, that start from 180 with his 78 micron mean diameter, and arrive at 1200 with 6 microns, but in the manufacture of mirrors usually uses to the maximum the W6 which has diameter 14 Micron and is virtually equivalent to the grain 800. their identification is infamously “criptics” with single-digit numbers that say nothing, preceded by a letter W; or simply by the number of minutes of sedimentation.

  • USE: Today this kind of abrasive used is less of a past time, and then they are also very hard to find. Were (and still are) recommended because less aggressive of the carborundum, and so it was easier to get a less rough in surface machining, as in those manuals, however, at the price of a much greater processing time.

Frankly and from insider, I think that today with manual processing (that already does not have the aggressiveness of the work with a machine) There is virtually no difference in perfect shiny (achievable of course with the usual pitch and cerium oxide or zirconium), on mirrors worked until the grit 800 carborundum, or machined to equivalent (for grain size) aluminum oxide grain W6.

INDUSTRIAL DIAMOND : It is pure carbon, hardness and 10 in the Mohs scale, that is the hardest known material  .

In the manufacture of mirrors powder is not used but sometimes amateur profitably be used for excavation of the curve on simple machines with rotary table, in the form of a cup or disc tools caked galvanically   industrial diamond powder.

CERIUM OXIDE: It 'a white rosy powder with grain around the micron, (There is also the rarest  OPALINE, that is cerium oxide powder further refined).   both are used during the polishing and subsequent parabolizzazione of mirror surface from “satinated” left from the grit 800 carborundum. it is’ used is no longer a mere interposition water between the two joint discs   mirror and glass tool, but with a tool covered by a layer of squares of pitch that must have only “partially yield” as a result of frictional heating given by processing.

This partial collapse of the pitch, and fouling in it ofCerium oxide, together with the presence of a small amount of water and   of the optimum working temperature (that is normally achieved after at least half an hour of tool strokes), are the essential ingredients to produce on the glass surface of the mirror, not only abrasion, but abrasion accompanied by a glass   molecular   "transport" clearly visible under a microscope (Like craters rounding edges, that prevoiusly where chipped, until their disappearance), which glass back locally fluid (at molecular level), for the heat-friction of the pitch and the other components, it go literally filling the craters and to pave the microscopic surface roughness, creating a super glossy like the surface of the water when it is pierced by sunlight incident rays, without making spurious reflections at the point of that incidence.

Polished that are unobtainable when abrasives (While infinitely small grain, and in the absence of his capture from a medium like the pitch), can roll freely between surfaces of tool and mirror.

ZIRCONIUM OXIDE: It is used in dentistry and orthopedic materials to fabricate dentures (false teeths, Coxe of femur etc). Reduced to a fine powder, Thanks to its cubic molecular structure (similar to that of the diamond) is stiff, and so fragile but abrasive in his pottery fragments with sharp edges (because its have ends with sharp edges “at molecular level”).

Personally I've used it for hand machining my mirror 300F6, but I have not noticed differences with cerium oxide. Indeed…: Cerium at some point in the processing engine created the beneficial foam in the water, able to circulate very efficiently the abrasive, and create that already named “Molecular transport” you want. The foam with zirconium oxide is not done independently, but must be caused by adding a bit of dishwashing liquid in water processing. But his biggest flaw remains is the lack of availability and with it the higher price than the cerium.

IRON OXIDE  (or also oxalate of iron) : The first is said “Rouge” - “-” or American slang “Rouge”. Abrasive   are "poor" in the sense of being much more common at one time, that they are less exotic than Cerium or Zirconium, Therefore since mid ' 800 widespread, and initially perhaps the only Abrasives used. Their lower abrasiveness required a longer and more accurate processing, providing though surfaces of a super glossy.

The big problem It have, and that has determined his abandonment, is that it get dirty of rust anything with which they come in contact. And this is serious because there is no way to clean dirty tissues and many furnishings   of rust.


The sequence is designed to refine the mirror surface with the progressive decrease of the abrasive grit dimensions. Then it is driven by halve the previous average grain size in microns, indicated in the tables of equivalence (Figure1).

Obviously the halve sequence is determined by the starting grit size, to get always and at least to end grit 800.

The halving is not peremptory, but is the method that provides the best performance, and because it makes it easy to see under a lens or microscope the disappearance of craters left over from previous coarse grits. Disappearance indicating that it is time to change grit.

But any grit in the vicinity of half the value of the preceeding grit, may be fine. At most only elongates work times, Since changing from a coarse grit to a too fine grit will require more Time to get rid of the big asperities.

An example of a sequence can be the following

Carborundum 60 80 120 180 240 400 800 Cerium oxide
Ø Microns 268 192 116 78 54 23 12 1- 1.5

Among those shown exist throughout the following series of other grits, You can replace one or more entries, Depending on the availability of suppliers of retail.

grit size 100 150 220 280 320 360 500 600 1000 1200
Ø Microns 156 93 66 44 36 28 20 16 9 6


Abrasives depend on the diameter of the mirror you want to achieve   and way of working for all those involved, and so it is not easy to indicate of reliable.

The mistake that we make more easily and frequently, is to use too much abrasive in each wet, in the intention of increase the efficiency of the excavation, but instead with so, optaining the practical opposite effect, to force this abrasive to “grind” himself in large quantities, and very less grind the glass, and with that, we hardly increase the working time and the costs in abrasive, limiting hardly the glass grinded, instead of speeding our work., and so wasting unnecessarily abrasive in his inactive self-destruction .

To determine the amount, until recently (i.e. before to make inconvenient ' DIY small mirrors, come to our markets low-priced Chinese mirrors) you could peep on catalogues of European suppliers, reading the composition of abrasive kit for DIY optical sale. And maybe what can still be done today, If from these suppliers someone is surviving (but the commercial dying has been great).

Reporting data from a European supplier that closed a few years ago.

Mirror diameter From 230 a 250 mm From 290 a 350 mm

From 360 a 400 mm

Stone Cote for chamfer

N° 1 N° 1

N° 1

Blocking Carbo 60

1.5 Kg 1.5 Kg

2 Kg

Blocking Carbo 80

1 Kg 1.5 Kg

2 Kg

Running or "wedding" Curves (equalization male and female) Carbo 120

500 g 500g

1 Kg

Carbo 180 (Emery W240 )

500 g 500g

1 Kg

Refining Carbo 240 (Emery W240)

250g 350g

  500 g

Carbo 400

150g 250g


Carbo 800 (Emery W6)

100g 100g


Carbo 1000 (Emery W8)

50g 50g


Polishing Pitch black Gulgoz #64

1 Kg 1 kg

2 kg

Cerium oxide

150g 150g


Opaline (Cerium oxide fine)

100g 150g


Carryover below the sequence that I have personally used in the processing of my 250F5.

After the successful completion of that mirror diameter, having enjoyed it,   I continued to use the same sequence of abrasives for subsequent work. The realization of the 250F5 started from a blank pre-hogged by machine from the supplier Reginato, and therefore excluded the use of coarse grains of that phase of the work.

The following are the recordings on my "journal of glass scratcher" concerning the manufacture of the 250F5 except parabolizzation that lasted almost as, with different "game over" ( returning back towards the sphere), with a consumption of cerium oxide by about 200 g.

Carbo   120 180 320 500 800
Consumed 0 71 g 82 g   25 g 29 g
No dried 0 70 85 28 28

  Average consumption of abrasive for each grain type ~ 1 gram to each wet
Total wets 211
Duration of each wet in minute 3

Total minutes of my hand work = 633         of     10.55 hours  (and about 633 laps around the Workbench).

At the rate of two strokes per second (1 stroke forward 1 stroke back) add up 75960 strokes manually performed (but the term stroke sometime is translate like "racing" from the ignoring automatic translator) …75960 is a good number! But your biceps are thankings instead of complaining !

REPERIBILITY’: for badinage, but in truth, we can say that in Italy these days we have a large quantity of all kinds of …”"grane"” …(in the sense of the italian corresponding term “troubles”), very different from the type abrasives.   the latter though, Unlike the first, are desirable goods, but very rare (in Italy) , species such as retail.

Suppliers retailers (is there a minimum retail sale in Italy, but is directed to processing of jevelery precious stones) be found on the network by entering in search entries, For example, “Facet for precious stones”or vice versa “cutting gemstones”; And within sites that sell those cars, they are pure consumables which they operate, that are “polishing powders” or cerium oxide or aluminium for the facet; as for the cutting machines it is powdered “Silicon Carbide”.

One of these Italian companies is called Gemmarum Lapidator .

New suppliers in Europe appear and disappear depending on the strength of the global crisis that engulfs, and usually sell these Abrasives only supplied with the purchase of a glass “blank” from work. A recent company that also sells separately is the French OAMS   optique astronomique.

A few photos:

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