I can assure you that adding caustic to hot water will instantly cause a very violent reaction. Always mix caustic with cold water. Always add caustic to water and not vise versa.

Hi EPJ,
John's question refer to adding water to cool down the solution as it has "burned"part of the water in it. The bath works at 275 to 295 F and since water evaporates at around 212, it will tend to "disappear" from the bath, lowering the level in the container and increasing the temperature of the remaining solution.

Thus, water must be added to the bath to continue the reaction at the desired temp range and to keep the level in the container where parts are submerged.

You are right, when water is added to the caustic bath there is a violent reaction. John was wondering if by adding hot water, instead of cold, it would lower the violence of the reaction.

I haven't tried adding hot water, since the process is complex enough to add another "layer' to it...but in the video I posted, you can see the gunsmith adding water from the boiling tank to the bluing bath...

hope this clarifies John's question and my answer.

I have home blued knife blades and some gun part's and have some more to blue. I made up my first solution, but then bought a pail of salt from Brownells. I have worked with caustics and other dangerous chemicals, steam etc for a lot of my adult life

It won't help much to add boiling water to the mixture over room temperature water. The problem is the rapid expansion when the water right as it becomes steam. One sq inch of water forms 1600sq inches of steam at atmospheric temperature. The amount of expansion from 70f to 212 f is very very small, but in that instant that it goes over the edge it goes up factor of 1600. Actually the cooler water should be less violent because it slightly cools the solution it touches and therefore SLIGHTLY slows down the forming of the steam bubble

The only good solution is simply add the water slowly and if you will notice in the video the guy is actually putting the water on the side of the container right at the edge of the solution. The container is as hot as the solution so he is causing his water to mix and any steam to form right at that edge which probably releases less solution in to the air as the steam flashes off. It also prevents the water from being injected beneath the surface of his solution and forming a large steam bubble under the solution and blasting it upwards.

If you have all your stuff mixed right and have an decent sized container, you shouldn't need to add much water. Get your parts in asap and try to keep your boil at the lower end and stave off adding water as much as possible. If you want to run more parts you can always allow the solution to cool below 212 ad water and heat it back up when the second batch is ready.

I have a some guns that need a bath, but don't want to do it until I finish some projects and they are all ready so I can set up do them one after the other

I have used a pipe full of special salt mixture super heated to 1500f to harden knife blades. At that temp it is a thin red liquid and of course has zero H20 in it. Nice thing about this method is once in the solution there is no 02 to burn carbon out of the surface of the steel, Go from there right to the oil and the blade looks just like when you stuck it in the solution once you wash it off. As it is a liquid the blade gets heated evenly everywhere and warping is very rare. The only problem is it is kind of dangerous (DUH) Anything you place in it had best be well over 212f for a while so it is bone dry when you insert it or KerBOOM and hot fire everywhere. You also have to cool it with a tapered rod stuck in it so when you heat it back up you remove the rod and the hole it leaves gives it a place to expand so you don't blow a big super hot plug of salt out into your shop. Be careful out there boys and girls

Where do we get rid of the used caustic water safely?

__________________
Plant garlic in the fall

Brownells sells HotSalt bluing neutralizing kit.
It's a bag of Sulfamic Acid (dry powder) basicly along with some Ph test strips and eyedroppers.
You have to supply Hydrochloric acid in the form of muriatic acid along with Sodium Hydroxide (Lye).

With these chemicals you bring the highly caustic salts to a ph neutral and disposal down the drain is supposedly OK.

It isn't cheap,,at around $250 IIRC.
You can buy just the bag of Sulfamic Acid w/o the do-dads and it's about $10 cheaper!

EPA is tough on those that throw this stuff away w/o proper disposal.
A large shop I worked in in the early 90's got fined in the multi 100'sK $$ for improper disposal of bluing salts. They also received fines for improper set-up in the bluing room that wasn't catching any splash over, poor ventilation, ect.
It didn't help that the wash down drains in the bluing room simply went under the ground and spilled out on the bank and into a stream that ran along the property.
EPA & DEC were not impressed!

The neutralized salts can't go into a septic system, or at least that what the rule used to be. It had to go into a sanitary sewer system so it was directed further to a municipal water treatment facility.