Tuesday, March 12, 2024


I recently invested in a hydrometer, which measures the density of a liquid relative to water. In etching, it is used to measure the ratio of water to ferric chloride in the etching solution, in degrees Baumé, or Be. Counterintuitively, the relationship is not simply one of more water diluting the strength of the solution; instead there is a useful range centered around 35˚Be, tailing off in either direction. The ferric chloride I buy comes as 42˚ Be, which is recommended for a “careful etch,” but other products I use were recommending 34˚, so I decided to adjust the Baumé with the hydrometer as a guide.  

gloved hand holding  hydrometer, showing calibrations from 0 to 70

The hydrometer is basically a closed glass tube, weighted at one end with what looks like birdshot and glue, with calibrations on a paper roll inside the tube.

Hydrometer floating in water, reading 0

Floated in plain water, it reads 0.

Hydrometer floating in ferric chloride, reading 42

 In the ferric chloride out of the jug, it reads 42. 

turkey baster, measuring cup, plastic jug, and etching tank

I gradually diluted the solution by removing ferric chloride with a turkey baster and replacing it with water 

Hydrometer floating in adjusted ferric chloride solution, reading 34

until it read 34.

Hydrometer floating in graduated cylinder with sample of ferric chloride, reading 41
Finally, I wanted to determine the Baumé of the solution I had removed, which had become slightly diluted in the process, and got about 41.

Etching of streetcar interior with pensive man in foreground, mother reading large book wile her daughter looks on, other passengers in background.

PCC Memories etched with 34˚ Be ferric chloride.

I don't know how much it will show up here, but it made a noticeable difference to etch with 34˚ Be ferric chloride - the lines I added were darker and sharper.

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