Sunning the bath and establishing the strength of the Silver Solution
Here is our experience with servicing silver baths over the years
No amount of filtering will remove the finest organic matter, nitrates and free iodides from a silver solution use in the wet plate process. Organic matter in the bath is one of the main causes of veiling on collodion plates. The other being impure or not properly filtered developer.
The silver solution will also contain acid, alcohol and ether after use. Sunning the solution in a large clear open container will remove the alcohol and ether by evaporation. Sunning works best if you bring up the pH a little toward the neutral side with bicarb before sunning. Sunning is the only effective way to remove all fine organic matter. It will not remove free iodides however. The solution requires the addition of more water and a boiling treatment for that, but if you are making tintypes or ambrotypes...you will not generally see the pinholes caused by free iodides. Negative making requires more "nicety" than positive making as a general rule and as you'll notice most people are making tintypes and ambrotypes these days.
Once sunned, so that the silver solution first turns dark and then clear again, you may filter the solution and test with a hydrometer to get a good enough test to show if you need to add water or silver to bring it up to the specs of a newly made silver solution. We never take note of the scales...just mark the shaft at the starting level with a sharpie as your bench mark.
The main thing to remember is that testing a used silver bath without sunning it first [to remove the alcohol and ether] shows you absolutely nothing..since hydrometers also measure the specific gravity of alcohol and ether. Very few wet plate photographers used titration in the 19th century, but a whole lot of them used hydrometers.
By the way, when we first came on the scene in the late 1980s...those few who were also doing wet plate, retired their silver baths completely when they showed signs of problems. Despite what they may tell you now..they stored gallons of them in back rooms and outdoor sheds. We have been using elements of our first bath since the very beginning and have always thought sunning was the best thing you could do to an over worked bath. We always test a sunned bath with a hydrometer...and strangely, have been able to make some pretty good images over the years, both positive and negative. ;-)