Use of Sal Ammoniac block

A block of sal ammoniac is an excellent aid to keeping your soldering iron tip (or bit) clean and able to hold a small blob of solder.

A description of what sal ammoniac is and the safety precautions in its use are here. 

• Place the block in such a way that it cannot slide around as you rub your iron over it.
• Place your hot soldering iron tip on the block until it begins to smoke. 
• Move your iron slowly back and forth along the block.  Initially, the block will be black from the contaminants coming from the soldering iron bit.  As you rub the bit along the block, it will begin to clear. As it does, you can add a touch of solder and turn the bit over to check whether there are still any black spots on the face of the bit. 
• If these spots are persistent, you can use a brass wire brush to help clean the contaminants off.  
• Add a touch of solder and return to rubbing along the block.  
• Repeat this check until the whole bit is bright and holding a small blob of solder.  
• This completes the tinning process on one side.
• Repeat this process for the other side too.
• Leave a small blob of solder on each side of the bit to protect the bit from oxidising.  

This cleaning process should be done at the end of each soldering session if the bit is not clean.   The frequent quick wipe of the bit on a damp sponge or a ball of brass swarf cleans the bit as you work.  The sal ammoniac block is for cleaning persistent contaminants off the bit.

Sal Ammoniac

There are sometimes concerns expressed about the use of sal ammoniac to clean the tips of soldering irons.  My conclusion is that there are no elements of the block that will affect the copper plating of the soldering iron bolt.  It is safe to use this as an occasional cleaning method of soldering iron bolts. This is based on the following information.

What it is

The common term, sal ammoniac, refers to the chemical ammonium chloride.  Sal ammoniac is the archaic name for it. The Romans named it from the ammonium chloride deposits that they collected from near the Temple of Jupiter Amun in ancient Libya.  It is found as encrustations around volcanic fumaroles, guano deposits and in burning coal seams. Notable occurrences include Tajikistan; Mount Vesuvius, Italy; and Parícutin, Michoacan, Mexico.

Wikipedia

Ammonium chloride is the product from the reaction of hydrochloric acid and ammonia.  Ammonium chloride is obtained as a by-product in different chemical processes.  It consists of white crystals that are also available in rods or lumps.  The substance changes directly from being solid to gas with no intermediate liquid state. The gas does not consist of ammonium chloride molecules but ammonia and hydrogen chloride. This shows that the salt decomposes easily. When stored, ammonia is continuously emitted and the substance gradually becomes more acidic.

https://www.fishersci.co.uk/shop/products/ammonium-chloride-99-6-analysis-acs-acros-organics-3/p-3586389

Safety

It is widely used in human medicines as an expectorant, diuretic, etc. and in veterinary medicines to reduce gallstones, so it is a relatively benign material in relation to human health. 

There are some hazards though.  It can cause serious eye irritation on prolonged exposure, and is harmful if swallowed.  The precautions are to avoid eating, smoking, and drinking when using it.  Use gloves and eye protection if you are using it for extended periods. If it gets into your eyes, rinse with water for several minutes. https://www.fishersci.co.uk/shop/products/ammonium-chloride-99-6-analysis-acs-acros-organics-3/p-3586389

It is highly soluble in water, and forms a slightly acidic solution. Its main characteristic that you need to protect yourself against is that it vaporizes without melting at 340 °C to form equal volumes of ammonia and hydrogen chloride gas. https://www.britannica.com/science/ammonium-chloride

The amounts of the gas are small when used to clean soldering irons, but as the gas forms hydrochloric acid in contact with moisture, you should use dust masks rated for inorganic acids.  The amounts are small and generally only cause sneezing and coughing upon contact.

The primary hazard is the threat posed to the environment. Immediate steps should be taken to limit its spread to the environment.

https://pubchem.ncbi.nlm.nih.gov/compound/ammonium_chloride#section=Top

Uses

In addition to medicine, it is used to clean soldering irons. It has uses in jewellery-making and the refining of precious metals.  Sal ammoniac has also been used in the past in bakery products to give cookies a very crisp texture.  In some areas, particularly Nordic countries and the Netherlands, it is still widely used in the production of a salty licorice candy known as Salmiak, or Salmiakki.  Formerly it was used as the electrolyte in dry batteries.  It has uses in fertiliser as a source of nitrogen mostly for rice and wheat crops in Asia. It is also an ingredient in fireworks, safety matches, contact explosives, cosmetics and many other applications.

https://thechemco.com/chemical/ammonium-chloride/

Conclusion

Although there are some mild safety precautions that need to be followed, there is nothing in the sal ammoniac block that can harm the copper coating of the soldering iron tip.