# silvering an AL reflector.



## jashhash (Dec 19, 2005)

Im considering making a graphite mold to cast silver reflectors. I have read a bit on mixing chemical solutions to silver glass mirrors but wonder if this method would produce results comparable to vacuum silvering. In anycase, how does one go about getting a reflector silvered.


----------



## HarryN (Dec 21, 2005)

I am not an expert in this area, but there are substantial differences.

In the case of cast metals, there is usually a lot of included oxygen in the surface, which dramatically decreases the reflectivity. It is useful to remember that silver oxide is black, and is used to make the "black" part of black and white film.

When a mirror or reflector are "silvered", usually it is done by evaporating the metal onto the surface under high vacuum conditions. Performed this way, the surface is initially nearly free of oxide. If left exposed, it will oxidize (regardless of which metal is used) so it is often overcoated with a thin protection of silicon oxide. Mirrors of course have the reflective coating on the "back" and reflectors have the coating on the "front".

If you want to make such a reflector yourself, I would consider using Chrome plating technques, or a shop that does chrome plating. It is also worth noting that after putting all of these fancy coatings on reflectors, many people end up dramatically decreasing the reflectivity of the reflectors anyway with all kinds of texturing.

I assume you have already considered Carly Lamps for their off the shelf reflectors. If you just need an uncoated one, their's are not a bad place to start. I think they have a $ 50 min order.

If you actually are casting the mirror out of silver, it might be possible to polish it enough to be quite reflective for a few days, so the challenge then is to find a protective coating.


----------



## Nubo (Dec 21, 2005)

Not sure if you mean to cast solid silver reflectors, or cast in Pyrex and silverize the glass?

Amtateur telescope makers produce perfectly serviceable reflectors by DIY silvering ("cold silvering") of glass mirrors. Polished silver is a great reflector; better than Aluminum, iirc. Of course, for a limited time only... 

http://www.textfiles.com/science/silver.txt


----------



## jashhash (Dec 22, 2005)

Would this cold silvering technique work for makeing reflectors?


----------



## Nubo (Dec 22, 2005)

jashhash said:


> Would this cold silvering technique work for makeing reflectors?


 
I would think so, if you are starting out with a clean pyrex glass "reflector". Telescope mirrors are, after all, parabolic reflectors. A flashlight reflector is just a steeper parabola. Since the article mentions you can have the telescope mirror facing either up or down, it doesn't seem that you'd have to worry about "catching" a precipitate on the steeply curved glass. Sounds like it is just attracted. Also, you wouldn't have to worry much about a too-thick coating like the telescope guys are. They have to be concerned with optical precision of a fraction of a wavelength. I think you could go with a thicker coating, which would give you more service life, assuming you are going to be polishing it once in a while. I don't have any idea how long it would take to tarnish given that it's right next to a heat source. I'm pretty sure I've read that old headlights used silvered reflectors and polishing was a maintenance item.


----------

