coating ionic filters at home

[rfcurry]

I had fun reading the long thread on using various gases to prevent oxidation on ionic filters. Several years ago I gave up putting cladding on exposed ionic surfaces. I looked into coating costs and availability. Then I pursued a method of coating the glass myself.

 

I thought about using glass as a coating. I remembered the Klondike gold rush when a fresh egg would be traded at the minehead for gold. In order to keep the eggs fresh they dipped them in waterglass, which sealed the eggs from bacteria, gases, and liquids. Pursuing that thought, I found studies done on using waterglass as a coating on glass and metal.

 

Waterglass (WG) is the common name for both Sodium silicate and Potassium silicate. WG is a staple in mineral paints used on concrete. It is a fascinating material and very inexpensive. I favor Potassium silicate. I've purchased from 2 suppliers, both offered a modest price and a pure product. You will want to thin the WG with distilled water. The thinning will depend upon the viscosity you favor, which will hinge upon the method of application. Obviously, the thinner the film, the better. WG will harden in a short time by evaporation; however, to get a good hardness and complete setting, you will want to use the kitchen oven and heat the filter gradually to 225F. See https://www.pqcorp.c...vrsn=7dc86711_3 for suggestions.

 

Once you drive ALL of the water out, the coating left is essentially Silica. Glass on glass. There are many ways to apply the liquid, but I will leave it to you to decide your favored method. You could even use a blade and an old turntable. Endless variety.

 

Their are lots of patents to provide a background, e.g., 4,578,100, WO 2011120656 A1.

 

Btw, when WG is applied to a porous ionic glass, the bonding is intense. However, I have found, to my regret, that a drip of WG on a dichroic filter surface bonds quickly to the substrate, cutting right through the sputtered coating. You aren't getting it off without removing the dichroic coating. (sigh)

 

https://shop.kremerp...cate-28-30.html

 

Enjoy!

[Andrea B.]

Reed, very cool post. Thank you for this intriguing idea. And the links.

 

(lordy, could I use waterglass on my fingernails to keep them from splitting when I'm outside falling over rocks trying to get a photo? ......j/k of course......)

 

And I loved the Egg Story.

[colinbm]

Good to see thanks Reed, I'll have to look closer at it.

[dabateman]

Interesting but this paragraph worries me:

"UV Transmission

Silicate films are generally stable to UV light exposure. Specially clarified

sodium silicate solutions, such as PQ’s E® silicate, transmit 92 to 98% of

light in wave lengths ranging from 430 to 700 millimicrons. Below 400 mil-

limicrons transmission drops off rapidly, exhibiting a value of approximately

40% at 325 millimicrons."

 

I like lots of deep UV and don't want the UV protection.

[ulf]

That is a very interesting idea.

 

When you want a very thin and even layer of liquid it is often distributed by spin coating.

That is/was the method used to distribute the protective lacquer when making CDs.

The disc to be coated got an excessive amount of liquid on the surface, then the disc was spun with a high RPM to get rid of most of the liquid.

The lacquer used foe CDs were often cured by UV as the next step.

 

The same principle, but much more controlled is used when processing big silicon wafers during production of IC-chips.

 

It might be possible to make some diy gadget that could be used for spreading WG thin and even.

If the layer is thin the drying time will be decreased and during spinning an airflow is created, speeding up the drying.

[rfcurry]

David,

I hear you. I don't want UV protection on UV bandpass filters either. Fortunately, a good Beers (Lamberts) examination would tell us that a 0.1mm, or less, Potassium silicate layer will not obstruct UV as much as a 0.5mm fused silicate cladding.

 

I had a transmission curve of KSiO2, but I can't find it. Google "potassium silicate transmission curve", the graph must be there somewhere. There are a plethora of studies with names such as "

Deposition and corrosion behavior of silicate conversion coatings on aluminum alloy 2024"

 

which you can peruse.

Cheers!

[rfcurry]

Ulf,

 

I was always fascinated by the Depo (Deposition) machines in the semi FABS. Hence my mention of the audio-turntable. There are so many methods of application of Potassium silicate to planar glass - dip, drip, spin, roll, blade - that I haven't tried all of them.

 

Cheers!