There's been some responses to my original posting so to answer a few questions and provide more detail:

The blanks do not have cracks (or other flaws such as crystal twinning) when they are built and should they develop cracks due to excessive drive later the crystal is ruined. The etching process does not leave behind the weakly attached debris of mechanical polishing. It will quickly remove the debris of mechanical polishing

Yes, I am using ammonium bifluoride crystals A friend gave me so I don't know where to buy them. In solid form the material is easy to store and relatively safe to handle while preparing the solution. The crystals are dissolved in water as needed to produce whatever concentration one wants.

I used Whink back in the 60's when I was a novice. It works. It contains stuff other than HF so make sure you clean the blank well. Actually, that applies to any method of crystal reduction.

I make up a solution in a plastic pill bottle and place a support structure for the blank inside. I use a cutoff piece of an old all-plastic hair brush placed bristle side up in the bottle. That way the blank can rest on the bristles and the solution can access both sides evenly. I usually swirl the bottle every few minutes to assist that process. Using plastic tongs and gloves I remove the blank, swish it through tap water bath #1 (change this bath frequently), then tap water bath #2, and finally distilled water bath #3. Then blot the excess water and place in the holder for testing. When the final frequency is achieved I use a final final wipe down of the blank with a Q tip soaked in isopropyl alcohol. I dispose of the small amount of depleted solution by first diluting it with water in a 5 gal plastic bucket.

Note that whether grinding or etching the material removal technique only applies to crystals that resonate in the thickness-shear mode, which includes FT-243, CR-1/A and most of the 1930's types. The former are AT cut and the latter are typically Y cut. I try to limit my frequency moves to about 25Khz or less (relative to 7MHz). Moving 100KHz in my humble experience often results in a loss of activity.

Dennis AE6C

On Sun, Jul 31, 2022 at 12:21 PM Dennis Monticelli <dennis.monticelli@gmail.com> wrote:

The book I have is "Crystal Clear" by Thompson, IEEE Press 2007. It goes into a lot of detail describing the transition from a cottage industry to the most extensive in the world during WW2.

As for the technique of stabilizing FT-243s, it is attributed to Bliley, although I wouldn't doubt that others discovered it as well because it was essentially discovered by accident. Bliley used HF etching for the final small move to the design frequency. That etch accomplished something else that was very beneficial. It got rid of the loosely attached bits of silicon oxide that were mechanically trapped in the surface. As the crystals were used in the field the vibration dislodged that material, which reduced the mass and caused the frequency to rise out of spec. Bliley was forced by the govt to surrender their Intellectual Property to other makers.

I have done a lot of surplus crystal etching using a buffered HF solution (don't use straight HF for God's sake). If folks are interested I could write down my learnings. I will share now that a quick dip in the solution will "clean" a blank far better than alcohol can because the aforementioned silicon oxide fragments clinging to the surface are easily removed by the etch while the well ordered crystal structure is resistant to the etch and doesn't reduce during the dip. The freq typically goes up 100Hz as the mass is removed. Q improves as well.

Dennis AE6C