Configuring and Testing My New Peristaltic Pump for DIY Silver Gelatin Emulsion Making

One of the smartest upgrades I can make in my silver gelatin emulsion workflow is better control.

When I add silver nitrate into salted gelatin, the rate of that addition matters. It is not just a convenience issue. It is a chemistry issue. That is why I decided to start working with a peristaltic pump. It gives me a way to make the silver addition more steady, more repeatable, and easier to test.

And in emulsion making, repeatability matters.

Silver gelatin emulsion making is a chain of controlled stages

What I want you to see is that silver gelatin emulsion making is not just one reaction.

It is a chain of reactions and controlled changes.

First, silver nitrate meets the halides and forms silver halide crystals. Then those crystals grow and change during ripening. Then unwanted byproducts are removed. Then the emulsion is brought to proper coating body. Then it may be chemically ripened further. Then it may be sensitized or finished with finals. Then it is filtered, coated, dried, exposed, and developed.

That is why a silver gelatin emulsion is both chemistry and craft.

The first reaction sets the tone for everything that follows

At the core of emulsion making is the formation of silver halide.

For a simple bromide emulsion, the reaction looks like this:

AgNO3 + KBr -> AgBr + KNO3

The silver bromide is the light-sensitive material we want. The potassium nitrate is a soluble byproduct that will later need to be removed.

This first stage is called precipitation, and it is where the emulsion begins to take shape.

The important point is this: the way the silver nitrate enters the salted gelatin changes the conditions under which those first crystals form.

That affects things like:

• crystal size
• crystal distribution
• speed
• contrast behavior
• density potential
• overall character of the emulsion

In plain terms, if the silver goes in too fast, too slow, too unevenly, or under poorly controlled mixing, the emulsion can behave differently.

Why the addition rate matters

When silver nitrate is added into salted gelatin, the chemistry at the point of mixing is changing in real time.

If the addition is controlled, the crystal formation is more predictable. If the addition drifts, surges, or varies from batch to batch, the emulsion can drift too.

That is one reason hand-poured additions can be tricky. Even with care, it is hard to pour at exactly the same rate every time.

A peristaltic pump helps solve that problem.

It gives me a controlled feed rate so I can say, with much more confidence, that this batch received the silver in the same way as the last batch.

That does not guarantee a perfect emulsion, but it removes one major source of inconsistency.

The key stages in the process

Here is the bigger picture in simple terms.

1. Precipitation

Silver nitrate reacts with the halides in gelatin and forms silver halide crystals.

2. Ripening

Those crystals continue to change. Their size and structure can shift depending on time, temperature, and chemistry.

3. Washing

Soluble byproducts and excess salts are removed so they do not interfere with the finished emulsion.

4. Remelting and finishing

The emulsion is brought back to coating consistency and may receive other finishing additions.

5. Coating and drying

The finished emulsion is filtered, coated, dried, and prepared for exposure.

6. Exposure and development

The coated material is exposed to light and then developed into a visible image.

Each stage matters, but the first stage is especially important because it creates the grain structure that the later stages work with.

Why I want the pump in my workflow

For me, the value of the pump is simple.

It gives me a better way to control one of the most important variables in emulsion making.

Instead of writing in my notes, “added silver slowly,” I can write something far more useful:

“Added 70 mL over 10 minutes at a fixed pump setting.”

That is real process control.

It means I can test one variable at a time and learn what is actually changing the emulsion.

My current target

In my case, I wanted to deliver 70 mL of solution in 10 minutes.

That means the target flow rate is:

70 mL / 10 min = 7.0 mL per minute

From my own pump test, I found that:

4 RPM = 4.6 mL per minute

That means each RPM gives about:

4.6 / 4 = 1.15 mL per minute per RPM

To hit 7.0 mL per minute:

7.0 / 1.15 = 6.1 RPM

So 6.1 RPM is my starting target.

That does not mean I trust it blindly. It means I now have a testable starting point.

How I test the pump

Before using the pump in real emulsion work, I want to verify it.

My basic test looks like this:

• use the exact tubing I will use in production
• prime the tubing fully
• run distilled water first
• set the pump to the target RPM
• collect the output for exactly 10 minutes
• measure the total volume
• repeat the test several times
• adjust slightly if needed

That gives me a real-world check on whether the pump is actually delivering what I think it is delivering.

And that is the heart of the whole idea.

Not guessing.
Testing.

Why this matters

Silver gelatin emulsion making is one of those fields where small changes can lead to very different results.

That is why tools like a peristaltic pump can be so helpful. They do not replace understanding. They support it.

The more I can control the precipitation stage, the better chance I have of learning what my emulsion is actually doing.

And that is where real progress begins.

Want the full premium version?

In the full Darkroom Diary Premium article, I go deeper into:

• the chemistry and theory behind precipitation and ripening
• why silver addition rate directly affects crystal behavior
• how this relates to grain size, speed, and emulsion character
• why washing locks in the results of earlier stages
• how I calculate and test pump flow rate in my own setup
• how to think about this as both chemistry and craft

If you want the deeper version with the full explanation and practical workflow, join Darkroom Diary Premium.

Darkroom Diary Premium gives you deeper analog learning and real darkroom insights, including premium articles, formulas, workflow notes, and practical guidance for handmade silver gelatin emulsions and other historic processes.