By Eric Moshier – Certified Heater Mason, Third-Generation Mason, MHA Technical Committee Member, ASTM E1602 Masonry Heater Group Member
This is the masonry heater curing schedule used by Solid Rock Masonry LLC of Duluth, Minnesota, taken directly from our current Masonry Heater Owner’s Manual. A newly built masonry heater cannot be fired at full load. The SR Core refractory castings, the firebrick, and the surrounding mortar all hold water from construction, and heating that water too fast turns it to steam inside the structure. Steam expanding inside a casting will pop or crack it. The masonry heater curing schedule has two stages: 14 days of air curing with every door and damper left open, followed by a sequence of 15 curing fires that step up from roughly 5 lb of wood to 35 lb, spaced 8 hours apart, and then continue climbing by 5 lb per fire until you reach the design fuel load for your model. Eric Moshier has been building masonry heaters for more than 25 years and sits on both the MHA Technical Committee and the ASTM E1602 committee. Follow this schedule on a site-built heater, an SR Core kit, a kachelofen, or any other masonry heater and the structure will last generations.
>> Download the Solid Rock Masonry Heater Owner’s Manual (PDF) – curing, daily operation, firewood, maintenance, and the full warranty.
What Is the Masonry Heater Curing Schedule?
It is the controlled process of driving construction water out of several tons of masonry before the heater is asked to do its job. Fire a green heater hard and the trapped moisture becomes superheated water, which can flash to steam faster than the masonry can vent it.
The damage from skipping this step is not always visible on day one. Hairline cracking, spalled casting faces, and popped firebrick surfaces often show up in the second or third season, and none of it is covered under warranty.
Do not use the masonry heater to dry drywall mud, and do not fire it while drywall mud is curing anywhere in the building.
Step 1: Cure the Masonry for 14 Days
Before any fire is lit, allow the heater to cure for 14 days with all doors and dampers open. Start counting after all masonry work is complete, including the finish material. Stucco, tile, and stone facings all add water to the structure, so the clock starts when the last of that work is done, not when the core is finished.
Leave the firebox door, the bake oven door, and every cleanout door open for the full 14 days, along with the combustion air intake and all dampers. Airflow is what carries moisture out of the masonry. Closing the heater up slows the cure down.
- Mist the brick, stone, or stucco facing 2 to 3 times per day with clean water for the first 3 days.
- If the house is not closed in, cover the masonry lightly with plastic to shield it from wind and blowing rain. Remove the plastic after 7 days.
- Store the glass doors somewhere safe during construction.
- Keep the room above 40F for the entire curing period. The heater can only be allowed below 40F after the drying and curing fires are complete.
Washing the exterior masonry
Within 1 to 3 days after the brick or stone facing is finished, wash the masonry to remove residual dust and mortar smears. Do not acid wash soapstone, kacheln, stucco, or plaster finished heaters.
- Dampen all masonry with clean water first.
- Apply approximately 1 part vinegar to 10 parts water.
- Scrub with a very stiff brush, then rinse within 3 to 5 minutes.
- Hard stone or brick: use a wire brush on stubborn mortar spots. Soft stone or brick: use a wooden dowel.
- Test a small area first and adjust the vinegar concentration as needed.
- If it is not cleaned within 7 days you may need Prosoco SureKleen 600. Follow the manufacturer instructions.
- Protect the steel doors with plastic and rinse thoroughly.
Efflorescence during curing is normal
You may see white, fuzzy, crystal-like growth on the brick and on the castings inside the firebox and cleanouts. That is mineral salt drawn to the surface as the masonry dries. It is harmless, but it will stain if it is left in place. Wash it off with 1 part vinegar to 10 parts water and rinse.
Step 2: The Curing Fire Schedule, Fires 1 Through 15
Once the 14 days of masonry curing are complete, begin the curing fires. Space every fire 8 hours apart and weigh the wood rather than counting pieces. Each heater firebox is designed to burn a maximum of a three quarter full firebox load by weight according to the model number, and you must not exceed the maximum rated wood load in any 8-hour period during or after curing.
| Fire number | Approximate wood load | Spacing |
|---|---|---|
| 1, 2, 3 | 5 lb each | Every 8 hours |
| 4, 5, 6 | 10 lb each | Every 8 hours |
| 7, 8, 9 | 15 lb each | Every 8 hours |
| 10, 11 | 20 lb each | Every 8 hours |
| 12, 13 | 25 lb each | Every 8 hours |
| 14 | 30 lb | Every 8 hours |
| 15 | 35 lb | Every 8 hours |
| 16 and up | Increase by 5 lb per fire: 40, 45, 50, 55, 65 lb | Every 8 hours, until you reach the design fuel load for your model |
Light every one of these fires from the top down, the same way you will light the heater for the rest of its life. A top-down burn is the fastest and the cleanest way to get a wood fire going.
>> Masonry Heater Use and Care – top-down lighting, loading, and damper operation
How Big Is a 5 lb Load of Wood?
A typical birch or maple log split to 3 to 4 inches in diameter and cut 16 inches long weighs roughly 3 to 4 lb. Weigh a few pieces of your own wood on a bathroom scale at the start so you know how many splits equal each target weight in the table. Species density varies enough that piece count is not a reliable substitute for weight.
Burn only seasoned cordwood under 20 percent moisture content for the curing fires. Wet wood will not reach combustion temperature, and a smoldering fire during curing deposits creosote in heat exchange channels that are not easy to clean.
Optional Stone Sealer After Curing
If you plan to seal your stone to bring out the colors, we recommend Prosoco (or White Cap Co. brand) Cure-N-Seal LV25, a concrete curing sealer with low volatile organic compounds. Apply it only after the facing has been washed and cured.
- Apply with a brush or sprayer. Protect steel doors with painter tape and plastic.
- Strong VOC odor for the first 2 to 3 days. Apply only when the windows can be open.
- Not all stone sealers tolerate the 180F to 200F surface temperatures of a masonry heater. Our clients have had very good results with this one.
- The shine diminishes after the first year but the color enhancement remains. Stone is a natural product and will take up sealer differently in different areas.
Do You Repeat the Masonry Heater Curing Schedule Every Fall?
No. A heater is cured once, when it is built. You never repeat the 15-fire sequence. What we do recommend each fall is two or three smaller fires at roughly one third of your rated fuel load, spaced 8 hours apart, before you go to full design load. An already-cured heater does not pick up enough moisture over a summer to be at real risk, so treat this as a cheap precaution rather than a repeat cure. It costs you a day and it removes any question.
Fall warm-up is also when the annual cleaning gets done. The heat exchange channels and the chimney should be cleared of fly ash before the first full firing of the season, not after.
>> Masonry Heater Maintenance – annual channel vacuuming and chimney cleaning
Masonry Heater Curing Schedule FAQ
How long does it take to cure a masonry heater?
14 days of air curing with all doors and dampers open, then the curing fires. At 8 hours between fires, the 15-fire sequence runs about 5 days, and a few more days beyond that if your model’s design load is well above 35 lb.
What happens if I skip the curing fires?
Water trapped in the castings and mortar turns to steam faster than the masonry can release it, and the pressure pops or cracks the refractory. Damage from firing a heater too fast during curing is not covered under the Solid Rock Masonry warranty.
Can I run the fires closer together than 8 hours?
No. The 8-hour spacing lets the moisture migrate out and the masonry equalize between fires. Running them closer together defeats the purpose of the schedule and counts as exceeding the rated load for that 8-hour window.
Do I have to weigh the wood?
Weigh it once at the beginning. Weigh a handful of representative splits, work out how many pieces make 5 lb, and count from there. Ten pieces of oak and fourteen pieces of pine can be the same weight, which is why the schedule is written in pounds.
What is the white powder on my new heater?
Efflorescence, which is mineral salt carried to the surface as the masonry dries. It is harmless and expected on a new heater. Wash it off with 1 part vinegar to 10 parts water before it stains.
Does a DIY kit build follow the same masonry heater curing schedule?
Yes. The schedule is identical for an SR Core kit built by an owner-builder, a heater built by a local mason, and a heater we build ourselves. Photo documentation of the build and adherence to the manual are both conditions of the DIY kit warranty.
Questions About Curing Your Heater
If something during curing does not look right, send photographs of the firebox, the outside of the heater, and anything you are concerned about, along with a description of where you are in the schedule. Most curing questions can be answered without a site visit.
Solid Rock Masonry LLC
5347 Howard Gnesen Road, Duluth, MN 55803
Phone: 218-343-2978
Email: eric@solidrockmasonry.com
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