Johnny Arvizu

My Soda Firing Process

Over the years, my soda firings have evolved through trial and error. I’ve worked through many different firing schedules, experimented with different clay bodies, and tested countless flashing slips trying to understand how each variable affects the final surface. Every firing teaches me something, and my process continues to evolve each time I load the kiln.

One of the reasons I’m drawn to soda firing is the number of variables involved. The stacking of the kiln, the work inside it, the clay bodies, flashing slips, the atmosphere, the stack, the amount of reduction, the timing of soda introduction, the amount of water, and even subtle temperature shifts all play a role in the surfaces that come out of the kiln. Because of these variables, I always change at least one variable in every firing. Sometimes it’s the clay body, sometimes the flashing slip, sometimes the atmosphere or the timing of soda introduction. Continually adjusting something allows me to keep learning and discovering new nuances in the surfaces.

Soda firing is humbling in a way that many other firings aren’t. There’s simply a higher loss factor that comes with this territory. Pieces can come out below expectations, shelves can get droppings from melted glaze or soda accumulation, and sometimes crust from the kiln ceiling finds its way onto the work. It’s all part of the process. Over time I’ve learned to appreciate that unpredictability and accept the losses as part of what makes the successful pieces feel so rewarding. Soda firing constantly reminds me that the kiln is never fully under my control.

The Role of Flashing Slips

Flashing slips play a massive role in my work and in how I approach soda firing. Many of the surfaces I’m chasing depend on how the soda vapor interacts with the clay surface. Flashing slips create the ideal conditions for that to happen.

Most flashing slips have finer particles and are more refractory than the clay body underneath, meaning they resist excessive soda buildup. Because of that, they create a surface where soda vapor can deposit and build layers over time rather than immediately melting onto the clay body. This encourages flashing, color development, and surface buildup that wouldn’t happen on most clay bodies alone.

In many ways the flashing slip becomes a sensitive canvas recording the time inside the kiln — where soda traveled, where reduction occurred, and how heat and vapor moved through the chamber.

Why I Love Soda Firing

I keep coming back to soda firing because no two firings are ever exactly the same. Even if I tried to repeat a firing perfectly, the kiln will always respond differently depending on the load, stacking, and subtle atmospheric shifts.

That unpredictability is what makes the process exciting for me. The kiln becomes a collaborator, and the final surfaces are the result of all those variables interacting together.

Because of that, I treat each firing as another opportunity to push the process slightly further and get lost in the sauce.

My Typical Soda Firing Schedule

Preheat / Candling

~8–12 hours

I start every firing with a long preheat, usually somewhere between 8 and 12 hours. During this stage I keep the burners low and allow the kiln to slowly warm up. The goal here is simply to remove any residual moisture from the work, kiln shelves, and posts while gently bringing the entire kiln load up in temperature.

A slow preheat helps prevent cracking, blowups, and keeps thermal stress to a minimum before the kiln begins its climb.

Climb to Cone 8

~10-12 hours

Neutral atmosphere

Once the kiln is thoroughly candled, I begin increasing the heat and start climbing toward cone 8. During this phase I keep the kiln in a neutral atmosphere ignoring body reduction, allowing the clay bodies and glazes to mature  evenly without introducing reduction too early in the firing. This allows for me to keep a brighter base color to add layers of darks later in the firing. 

While the kiln is climbing to cone 8, I prepare and sieve my soda solution in two batches and prepare both of my sprayers. I make two small batches because I take so long during the introduction phase that the soda starts to crystallize and clog the sprayer.

My main goal during this climb is to keep the kiln even in temperature from top to bottom. 

Soda Introduction

~1 hour

Cone 8 → Cone 10 

When cone 8 begins to bend, I start introducing soda into the kiln.

I spray soda solution into the kiln in short bursts of about 15–30 seconds. After each soda spray I follow with water, also sprayed in 15–30 second increments.

The water serves an important purpose in my process. It helps tone down the heavy gloss that soda can create and encourages more crystal growth along with satin and matte surfaces rather than overly glassy ones.

After each spray cycle I wait for the kiln atmosphere to stabilize and for the temperature to recover back to where it was before spraying. Once the kiln regains that temperature, I wait for it to climb another 2–5°F above the previous temperature before spraying again.

This rhythm of spraying and waiting helps keep the kiln balanced and gives the soda vapor time to move through the chamber and reach more work evenly. 

Layering the Atmosphere

Layering plays a massive role in the surfaces I’m trying to achieve. That’s one of the main reasons I introduce soda in such short increments rather than large dumps.

I start my spraying cycle in oxidation and maintain oxidation for four sprays. After the fourth spray I switch into heavy reduction spraying once. I repeat this oxidation/reduction cycle until I sprayed all the soda solution and water. My goal with this layering is to encourage different atmospheric conditions to build on top of each other. 

Ideally this creates white soda buildup layered over areas of carbon-trapped soda, giving surfaces a sense of depth, complexity, and a shift in surface feel. 

This spraying cycle continues as the kiln climbs from cone 8 to cone 10, usually taking around an hour until cone 10 is soft and all of the soda has been introduced.

Oxidation Downfire

~1.5-2 hours

Cone 10 → 1900°F 

Once the kiln reaches cone 10 and all soda has been introduced, I begin a slow downfire in oxidation until the kiln reaches around 1900°F.

This stage usually takes between an hour and a half and two hours. I keep the atmosphere oxidized and slowly cooling during this time to help promote crystal buildup.

Hold at 1900°F

~1 hour

Once the kiln reaches 1900°F, I hold it there for about an hour. This soak allows the glazes and soda deposits to stabilize and helps even out the temperature throughout the kiln.

Heavy Reduction Downfire

~4-5 hours 

1900°F → 1650°F 

After the hold, I shift the kiln into heavy reduction and downfire to 1650°F. This cooling stage usually takes four to five hours.

The deeper atmospheric effects develop during this stage. The slow cooling in reduction helps encourage rich iron responses, deeper flashing, and complex surfaces across each piece.

Final Oxidation and Shutdown

~1650°F

Once the kiln reaches 1650°F, I briefly shift the kiln back into oxidation.

I let the kiln run this way for about ten minutes. This short oxidation burst helps brighten some of the iron colors and burn off excess carbon, bringing out a little more variation in the final surfaces.

After that, I shut the kiln down completely and allow it to cool naturally. I try my best not to peek into the kiln too often at this point.

In many ways this schedule is still evolving. Each firing reveals something new, and by adjusting one variable at a time I’m able to keep exploring what this process can do. The kiln becomes a record of those decisions, and the surfaces that come out of it tell the story of everything that happened inside.

Randy Johnston’s bisque Flash

• Nepheline Syenite - 30%

• EPK (or whatever is not extinct) - 10%

• Grolleg - 50%

• Newman red (again not extinct iron rich sub)- 10%

Arvizu gold slip (ball milled)

• Grolleg - 52.63%

• Neph Syenite - 31.58%

• Tile 6 - 10.53%

• Locally harvested clay (iron rich) - 5.26%

Amber celadon

• Custer Potash Feldspar — 24.57

• Silica — 22.33

• Whiting — 15.15

• EPK — 19.32

• Talc — 5.02

• Bone Ash — 1.46

• Strontium Carbonate — 3.60

• Frit 3124 — 3.60

• Titanium Dioxide — 2.23

• Manganese Dioxide — 2.70