Where the
world gathers for metal finishing
Q&As since 1989


-----

Silver on glass

We have a requirement to produce a high quality reflective silver surface on a glass tube. The silver would be applied to the outside but reflection would take place on the inside (same as your average glass mirror). Ends of the tube to remain uncoated and both the inside and outside would be exposed to a glycol/water coolant mixture. Any suggestions for a suitable coating process most welcome. Sufficient adhesion to resist coating being lifted off by the coolant could be a problem. Numbers are small i.e. twenty to thirty tubes.

Graeme T [last name deleted for privacy by Editor]
- Edinburgh
1999

Graeme- In the 60's, NASA used a gold tint on the inside of spacesuit visors as filters (ever wonder why they looked so reflective from a distance?). This was produced by a very old process known as "silvering", which has been used to deposit nickel, gold, copper, and silver. See "Metal Finishing Guidebook and Directory" under "metallizing non-conductors". At least, that's where it used to be. I don't know about the newest ones. I personally have experience only with silver. This process utilizes a double-headed spray gun which sprays the metal-containing solution from one head and a "reducer" from the other head, forming a metallic film on the (properly prepared) target when they meet. This is a VERY EASY process to master, but does require the proper equipment. I don't know about adhesion, but if I were you I'd probably look into some sort of coating to apply over the silver, just as they do with mirrors to protect the (very thin) film. Patrick Marks

Patrick M [last name deleted for privacy by Editor]
- Chandler, Arizona
1999

Graeme- IVD Aluminum can be applied on glass. You get a mirror like finish on the inside. For abrasion resistance, I recommend that Hard Anodize be applied on top of the IVD Aluminum. These coatings and the required masking are not cheap.

Jerry Wahlin
plating shop - Compton, California
1999

---

Chemical silvering is used for mirror coatings but is protected by a topcoat because the adhesion is not too good. PVD silver with a Cr or Ti basecoat has better adhesion. Silver is easily corroded and PVD aluminum is a better choice for corrosion resistance (they make aluminum auto radiators) if there is no chlorine in the coolant. Chromium is very good for corrosion resistance but you only get about 60% reflectance. Don Mattox

Donald M. Mattox
Society of Vacuum Coaters
Albuquerque, New Mexico 1999

We have a sputter system that can be used to deposit a highly reflective and durable coating on our glass. Either silver or aluminum can be used, and then a protective oxide layer can be deposited. Of course a very thin Ti adhesion layer would go down first. We are limited to a substrate size of just over six inches -- how long are your tubes?

Joe Ketterl
- Issaquah, Washington
1999

---

February 5, 2009

Hello,

I was just reading the post about a gentleman who needed a mirror coating on the outside of a glass tube. WE have a similar application, but the requirement is to coat silver or another high-reflective coating on the INSIDE of a 25-30mm glass tube. The coating will only cover about 1/2 of the profile of the tube.

The tube will house a lamp and be hard-sealed and evacuated. length will be from 6" to maybe 18" max.

Here is my thought: Apply the silver to a small-diameter heating cartridge element, make a jig in which it fits inside the glass tube, and is sealed to the vacuum system. Power applied, silver melts and then vaporizes onto the glass.

Since the tube will be evacuated in a matter of an hour post-silvering, I am assuming that no protective over-coat will be necessary.

Possible problems: Glass will get too hot? Impossible to mask the tube at those temps?

I know chemical silvering is an option, but it seems to me to be quite expensive and labor intensive.

This is my first post on this forum--looking forward to your responses!

Garett Churchill
product designer - San Diego, California, USA