Se gentile richiesta di un utente del forum, ripropongo questo mio topic già pubblicato su di un'altro forum...
E' un argomento tra l'altro già parzialmente trattato, mi sembra, dal nostro Cicci.

Piccolo escamotage per poter riconoscere quando un produttore riesce a prodursi le viti in house , particolare considerabile "minore" nella costruzione o rifinitura della totalità di un calibro ma che potrebbe essere spia della raffinatezza ed attenzione al particolare dimostrando la reale volontà di essere "manifattura".

Le viti in acciaio fornite dai fabbricanti di calibri "terzisti" come Eta o similia sono spesso (a secondo del grado di finitura del kit) in acciaio ricoperte al palladio o nickel, questo trattamento le protegge dall'ossidazione e le rende più lucide, se si tenta di brunirle portandole in ossidazione a circa 300 gradi non accade nulla perchè il trattamento superficiale impedisce alle viti stesse di ossidarsi.

Per poter dunque brunire viti fornite con il kit del calibro bisogna lucidare le teste e poi effettuare il trattamento di ossidazione, il risultato sarà molto probabilmente che lo "spacco" della vite (a meno che non si sia riusciti a lucidare anch'esso asportando il palladio/nickel ed allargando lo spacco stesso) e la filettatura (questa sicuramente) rimarranno "bianche" perchè in quei punti non è stato possibile asportare il trattamento iniziale.

Chi invece riesce a prodursi anche le viti in house, riesce a brunirle totalmente proprio perchè questa non viene trattata al palladio prima del processo di ossidazione.

Ecco delle viti made in Glashutte Original, viti che oltre ad essere "in house" (notare la filettatura brunita) sono scelte a mano a secondo del colore per avere lo stesso punto di blu per ogni specifico calibro sul quale verranno montate:



Le viti vengono manualmente lucidate a specchio da un'impiegata G.O. che sembra riuscire a "finire" circa 60 viti al giorno:



Quì si intravedono alcuni viti, prima e dopo il trattamento di brunitura:



Aggiungiamo poi anche che una colorazione simile a quella ottenuta per ossidazione (ma non uguale) si ottiene anche con un processo chimico, utilizzando degli acidi specifici (come il selenico/selenioso) per lo scopo.

Processi di colorazione questi, utilizzati sulla quasi totalità di orologi di basso costo.

Addirittura si possono ottenere anche colorazioni similari (anche per lancette ed affini) utilizzando prodotti coloranti disponibili in paste, vernici, ed addirittura "penne coloranti".



Un'interessante trattato estratto dal "The British Horological Institute" confidando in una vostra probabile dimestichezza con la lingua inglese

General
Some of the methods are very dangerous and should only be undertaken with extreme caution. All the methods use heat and will give off some fumes. These methods should only be used in a well ventilated area and away from any younger persons who may get hurt. The BHI does not endorse any one method of bluing hands, but suggest you try some of these methods to find which one suits you best.

Many clocks and watches that come into the workshop for repair have blued hands that are not in as good a condition as they could be. It is, with practice, quite a simple job to re-blue the hands again. The process involves cleaning the old hands and then heating the hands until they are the right colour.

If the rust on your hands is only superficial, try placing them on the bench and scrubbing them with a steel brush (hardware store variety, fine wire). It will remove the rust and leave the basic heat blued original finish. A coating of oil or wax will make them look good and help to inhibit further rusting. However if the rust is more than superficial, you will need to undertake the following.

Preparation
It cannot be emphasised enough that the final finish depends on the amount of polish and finish you put into the hands before you blue.

If you are re-bluing existing hands and the hands are old and rusted, first remove the rust by placing them on a piece of pith wood with the tube pushed down into the pith wood to hold them in place and remove the rust. You can use soap stone, or they can be buffed with extreme care or diamatine (diamond dust) and oil on a hard wood stick flattened on one side.

To avoid the need for polishing away the blue to obtain the bright polished finish required for a good blue job, you can boil the hand in a mixture of boric acid and water. This will remove the blue without the need for abrading it away. Also, quite often the finish remaining is good enough that re-polishing is not required, or at least minimised. Be sure to remove all wax before boiling.

When making a new hand , it is best to avoid the use of tool steel. It often contains elements that prevent bluing by heat. A simple heating of a test piece of the material will save a lot of wasted work. Also, when making a hand, one sometimes gets a poor result and needs to start the bluing process over again. Of course, the quality of the blue obtained is very dependent on the polish and cleanliness of the hand before bluing. When making a new hand, I usually go from a file, through the emery grits from 320 to 600, followed by buffing with Tripoli and rouge, then a solvent to remove the grease of the polishing compounds. At this point one is wise to use cotton gloves to prevent finger marks. I once had a pair of dirty tweezers ruin a blue job.

Bluing
If you want to do the job properly, heat bluing is the route for you. But, be prepared for a lot of practice and frustration. The thinner the metal you are working with or the degree of taper from boss to tip that you are bluing, the greater the difficulty.

Too much heat, and you will "white out" the steel, and will have to start all over. Not enough movement of the heat source according to the thickness of the hands and you will vary the colour from too light to deep blue to purple, to brown. Whenever you "white out" you have to polish the steel down again, and start over.

Method 1.
This method involves placing the hands on a bed of granules that will transmit the heat evenly over the whole length of the hand.

Place granules (silica sand, Salt or Clean Brass filings) in a tin or metal type pan. Use about 1/8" of granules in pan. Lay the hands finish side up on the sand and with heat source of some kind such as a wood oil burner, begin heating the bottom of the pan and keep the pan moving with the hands staying in the centre of the flame as much as possible. After the sand gets hot the hands will begin to change colour - a reddish colour first and a maroon colour and then watch carefully for the colour you desire. When the colour is as you please, remove the hands and place in water or oil to cool.

NOTE: Some people use molten Lead or Hot Oil instead of granules. Be aware that hot liquids can do more damage to you than hot metals if you should accidentally tip them over yourself (or worse someone else!)

Method 2.
Use a hot air gun, you know the sort of thing that’s used for stripping paint. With a little practice (very little, I might add) this gadget enables almost perfect control of the process. By varying the distance of the nozzle from the hand (which is placed on a heat resistant fibre board) it is easy to progress slowly through all of the colours until a nice even blue is obtained. Initial preparation of the hands is of course the same as for any other method of bluing.

Method 3.
An electric tempering oven with a pyrometer and temperature control enables you to set any temperature in the range of colouring or tempering steel. By putting the hands in the oven and then letting them come to temperature, they will blue themselves. It is also useful for tempering pinions and other clock parts, for annealing arbors to be drilled, and for some kinds of soldering, etc.

Method 4.
For cheap clocks where the full heat bluing process would be an overkill, strip them and use selenic acid, obtainable as gun blue under the trade name (Formula 44/40) It gives a blackish colour, acceptable in cheap clocks whose hands were probably finished similarly, and will last with a faint coating of oil, wax or lacquer

Touching up afterwards.
When bluing on brass shavings, or clean sand, some parts of the hand come-up more quickly than others. The lagging portions can often be brought up to the same colour as the darker parts by either of the following 2 methods.

Method 1.
Using a slightly curved piece of 1/4" X 1" brass strip. It can be clamped to a stand over either a controlled Bunsen burner flame or (for a long time) over an alcohol lamp. The area to be blued can be rubbed over the heated brass while the hand is held in tweezers. As colour progresses move the hand along starting at the thickest or widest part and moving to the thinner areas in need of repair.

Method 2.
By waving the pinpoint flame of a small torch across the lagging part. "Wave" is the secret. Just one fast pass across the lagging area and wait, then another and wait, etc.

Pseudo Bluing
It is possible to simulate bluing for clocks and watches that do not justify the cost of proper bluing.

The substances available are:-

Blue enamel (sold for the purpose),
Gun bluing paste
Super Blue Extra Strength Gun Blue For Steel
Brownells Oxpho-Blue (USA)
Parker Hales's Comet Super Blue.(UK)
Note: The above contain various acids including the poisonous Selenious acid.

Paint
Made in the U.K. called Citadel Colour and its colour is deadly nightshade. Brush an extremely small amount on and I doubt that anyone could tell that it was retouched. I'm confident that if any blued hands needed repair and I needed to hide a small amount of Tix solder, that this combination will do the job nicely.
I think the Citadel range are of a similar formulation to artists' acrylic colour, with some being opaque and others (perhaps "deadly nightshade") translucent.
The firm Citadel Miniatures AKA Games Workshop Inc. is UK based, The addresses that I have for the companies are:

Games Workshop Ltd, Chewton Street, Hilltop, Eastwood,
Nottingham, NG16 3HY, U K
Tel: (01773) 760462
and
Games Workshop Inc., 3431 C Benson Avenue, Baltimore,
Maryland, 21227-1072, U S A
Tel: (301) 644 5699

A selection of marker pens
"edding 800". It is more opaque and a better colour than other markers that I have tried, and the result is not as black as gun blue. The only problem that I encountered was that it did not dry very hard, and was prone to fingerprinting. A satisfactory solution is to let it dry and then quickly brush on a coat of auto acrylic lacquer -- this results in a hard durable surface. Make sure you clean your brush well afterwards as some of the blue colour will almost certainly have been picked up.
'layout blue'.
A quick swipe of the felt tip covers any piece of metal, brass or steel, with a bright blue
coating, which dries instantly, and shows lines made by a scriber with superb clarity - far easier and more convenient to use than old fashioned marking fluid.
A couple of words of caution - there are two versions of these pens - water based and spirit based, and it is the latter which is useful. The spirit dries out very quickly, and it is essential to keep the cap tightly on when not in use. I store mine upright, cap down, and this seems to make them last longer. After the work is complete, all traces of the blue are easily removed with methylated spirit (denatured alcohol). We use these markers on a regular basis in the BHI seminar workshops.


Bluing Screws
To blue a screw head you first have to clean it up, polish it, and then remove any oily or greasy deposits from the surface.
holding the screw (by its thread) in a pin vice or lathe; cleaning the slot with a piercing saw or junior hacksaw; removing any burr with a pivot file
polishing the head on a piece of crocus paper or 3/0-emery, supported on a cork or other flexible sheet.
When the polish is to your satisfaction, wash it off in meths, alcohol, or other volatile solvent.
Then, holding it by the threaded end in a pair of old snipe-nose pliers, slowly pass it in and out through a spirit lamp flame. Watch very carefully for the straw colour beginning to appear and continue until there is a nice deep purple. By this time, with a little experience, you would already have the screw withdrawn from the flame.
As soon as the colour is right *immediately* quench the screw in water, dry it and rinse again in solvent.
Complicated? -- the whole operation would probably be completed in less time than it took to read this paragraph, and it is FUN.

If you have a *whole batch* of screws to get to exactly the same colour, take a piece of thickish brass plate, drill a number of holes clearance size for the screw thread in it, and drop a screw into each hole.
Heat the whole plate, screws and all, over the alcohol lamp, then when the right colour is reached (or just a smidgen before), upend the plate and tip all the screws simultaneously into cold water. I dewater the screws in white spirit, then dry (with a soft tissue, not a hot air gun - this could change the colour), and give a final wipe with thin oil to bring up the blue colour to its gleaming best, and stop any future rusting.

Alcune immagini che riguardano le macchine utilizzate per il processo in JLC.
(courtesy tjn "The purist")

Maison attenta e con notevole know-how nel campo della colorazione e trattamento dei metalli.



Vaschetta per le viti con l'olio utilizzato per il processo:



Controllo visivo:



Le varie colorazioni ottenibili a secondo della temperatura utilizzata, mostrate da Stefanie Smithuis (caruccia tra l'altro [] ) :

Notare come sia possibile anche ottenere diversi colori come alcune tonalità di rosso (utilizzato anche su lancette come quelle, ad esempio, viste su alcuni chronoswiss)



Risultato finale:



Per finire notare l'attenzione dell'impiegata JLC seduta a sinistra [}:)]



Senza prendermi alcuna responsabilità sull'eventuale esplosione della vostra casa [] oltre ad aggiungere che sistono prodotti già pronti per l'uso ecco comunque una ricetta per realizzare in casa dei
"Blueing salts" per effettuare una colorazione chimica dei particolari in acciaio.

Nitrato di ammonio 33%

Idrossido di sodio (soda caustica) 33%

Acqua distillata 34%

La mescola dei componenti deve avvenire esattamente come descritto, nell’esatto ordine, ed usando maschere protettive, occhiali e guanti.

(1) Preparate la giusta dose d’acqua e versatela nel recipiente, quindi, lentamente, aggiungete il nitrato di ammonio, fino a che lo avrete versato tutto.

(2) Ora aggiungete la soda caustica. Questo componente DEVE ESSERE VERSATO MOLTO LENTAMENTE poiché la soda caustica potrebbe reagire con l’acqua e produrre molto calore, ed il nitrato di ammonio libererebbe gas di ammoniaca, causato dall’ebollizione, se la soda caustica viene versata troppo velocemente.

Una volta che i tre componenti sono assieme, la miscela può essere riscaldata e dovrebbe bollire ad una temperatura di circa 150° C.
Se la miscela bolle ad una temperatura inferiore, bisognerà aggiungere soda caustica, se invece bolle ad una temperatura troppo elevata, si aggiungerà acqua. Ovviamente bisognerà compiere questa operazione molto lentamente.
Una volta portata in ebollizione la miscela non rilascerà più odori di ammoniaca.

Infine delle belle Macro provenienti da una calibro IWC fornitemi dal nostro amico Renato di orologeria.com









Credo che esplichino molto meglio delle parole alcuni dei concetti espressi in questo topic...

Aggiungo inoltre un'interessante articolo sempre sulla lavorazione e finitura delle viti anche se non specifico sul loro trattamento di ossidazione o colorazione.

http://www.horlogerie-suisse.com/articles/...gerie-1907.html

(testi di Caroline Sermier su iniziativa del caro amico e maestro Giulio Papi).

Enjoy

Lory