Refractory dry-out is the controlled first heating of a new or repaired castable lining, slow enough that its water leaves as steam, about 1,600 times the water's volume, without building more pressure than the lining holds (HWI, 2020). The schedule comes from the producer of the installed product, not from one universal standard.
Dry-out applies to castables, gunning mixes and other water-bearing monolithics, not to fired brick. Castables are one family of refractory materials, whose definitions and classes start under refractory materials.
A castable holds 4 to over 40 % mixing water; free water leaves at 100 °C (212 °F) and combined water en masse at 200 to 320 °C (392 to 608 °F) (Hotwork, 2002). Producer guidance asks for a 24 h cure at 21 to 32 °C (70 to 90 °F), a 56 °C/h (100 °F/h) ramp and a hold at 649 °C (1,200 °F) of 1 h per 25 mm (1 in), which is not an API Std 936 requirement.
Every value names its standard and edition, the product data sheet or the dated source it comes from; see our methodology and fact-checking process.
What Is Refractory Dry-Out and Why Is It Required?#
Refractory dry-out is the first controlled heat-up of a monolithic lining, and it is required because a cured castable still holds free mixing water that turns to steam in the first heating and must escape before service temperature. A conventional castable needs only about 25 % of its added water for hydration (Jenkins et al., 2011, Sulfur Recovery Symposium paper).
Plant and contractor documents also call the step a bake-out, and the rest of the mixing water leaves during it.
Curing, Dry-Out and Heat-Up: Three Stages With Different Limits#
Curing is hydration at ambient temperature during the first 24 hours or more, dry-out is the controlled first heating, and heat-up is every later rise to operating temperature. In refractory practice each stage has its own limits, and the other steps of a reline are set out under refractory practice.
HWI Schedule 2 PLUS (sheet dated 2013-12-17) writes the stages as separate blocks: a cure of at least 24 h at 21 to 38 °C (70 to 100 °F), a dry-out ramp of 56 °C/h (100 °F/h) with a hold at 649 °C (1,200 °F), and a reheat of a dried lining at the same rate.
Why Steam Pressure Causes Explosive Spalling#
Explosive spalling happens at the first heat-up when steam pressure inside a still-wet lining exceeds its tensile strength and the hot face breaks away. A faster ramp releases more steam per hour into the same pores.
Water expands about 1,600 times in volume as steam (Jenkins et al., 2011), and the steam escapes only through the pores. Dense low-cement castables have the lowest permeability after air curing and the most water-release problems in the Jenkins family table. Explosive spalling is one of several refractory failure modes, and the others are compared under refractory failure modes.
Who Sets the Schedule: Producer Sheets, Trade Practice and API Std 936#
The schedule for a refractory dry-out comes from the producer of the installed product, the installer or designer turns it into a written plan, the owner approves it and a dry-out contractor runs it (Jenkins et al., 2011). Third-party burners replace the main burner where its turndown is insufficient.
Producer Schedules Govern the Installed Product#
A producer schedule such as HWI Schedule 2 PLUS (dated 12/17/13) applies only to the products rated for it, here castables and gunning castables dried from one side. The producer's standard schedules cover single-component linings of 229 mm (9 in) or less; over 305 mm (12 in) the producer is consulted.
The hold of 1 h per 25 mm (1 in) is producer and trade guidance (Schedule 2 PLUS; Bases, 2010), never an API Std 936 requirement. Because refractory castables differ in binder and water demand, product classes are covered under refractory castables.
What API Std 936 Covers, Including the Dryout Index#
API Std 936, 5th edition (March 2024), is the quality control standard for monolithic refractory linings, and its scope covers material and installer qualification, production sampling and dry-out.
Its Dryout Index table applies to conventional castables only and is never a schedule for low-cement castables, which dry on their producer's schedule even where API Std 936 governs quality control.
Refractory installation quality control under API 936 extends to inspector qualification, and scope and inspector rules are summarized under API 936 refractory installation quality control.
How Much Water a Castable Holds and When It Leaves#
A castable holds 4 to over 40 % mixing water depending on its family, and that water leaves in two steps: free water at 100 °C (212 °F) and combined water en masse between 200 and 320 °C (392 and 608 °F) (Jenkins et al., 2011; Hotwork, 2002).
Table 1 compares five castable families from one published table (Jenkins et al., 2011; metric calculated).
| Castable family | Bulk density, kg/m³ (lb/ft³) | Mixing water, % | Permeability after air cure and water-release problems |
|---|---|---|---|
| Low cement | 2,243 to 2,883 (140 to 180) | 4 to 8 | lowest permeability, most problems |
| Conventional | 2,082 to 2,723 (130 to 170) | 8 to 12 | low |
| Medium weight | 1,602 to 2,082 (100 to 130) | 15 to 18 | moderate |
| Insulating | 481 to 1,602 (30 to 100) | 20 to 40 | high |
| Ultra-light insulating | 320 to 481 (20 to 30) | over 40 | highest permeability, fewest problems |
Table 2 compares the temperatures at which each form of water leaves.
| Water form | Temperature, °C (°F) | Source |
|---|---|---|
| Free water | 100 (212) | HWI, 2020 (producer guidance) |
| Cement hydrates (HWI points) | 227, 277 and 549 (440, 530 and 1,020) | HWI, 2020 (producer guidance) |
| En-masse release of combined water | 200 to 320 (392 to 608) | Hotwork, 2002 (trade article) |
| Critical release of combined water | 204 to 316 (400 to 600), sometimes up to 482 (900) | Jenkins et al., 2011 (conference paper) |
| CAH10 and alumina gel | 100 to 130 (212 to 266) | thermal analysis paper |
| C2AH8 | 170 to 195 (338 to 383) | thermal analysis paper |
| AH3 | 210 to 300 (410 to 572) | thermal analysis paper |
| C3AH6 | 300 to 360 (572 to 680), then about 450 (842) | thermal analysis paper |
| Boehmite | 530 to 550 (986 to 1,022) | thermal analysis paper |
Water Content by Castable Family and Density#
Low-cement castables take 4 to 8 % water at 2,243 to 2,883 kg/m³ (140 to 180 lb/ft³), conventional castables 8 to 12 %, and insulating castables 20 to 40 %, according to the family table of Jenkins et al. (2011). Insulating castables thus carry five times the water fraction of low-cement ones.
Because a conventional castable needs only about 25 % of its added water for hydration, about three quarters stays in the pores as free water that the dry-out must remove (Jenkins et al., 2011).
Free Water, Hydrate Water and the 200 to 320 °C Window#
Free water leaves at 100 °C (212 °F), while the water bound in cement hydrates leaves at higher temperatures that HWI gives as 227, 277 and 549 °C (440, 530 and 1,020 °F). A cement-bonded lining therefore keeps releasing steam long after its hot face passes the boiling point.
Two published windows describe the main release of combined water. A trade article by Cobane (Hotwork, 2002) puts the en-masse release at 200 to 320 °C (392 to 608 °F); Jenkins et al. (2011) put the critical range at 204 to 316 °C (400 to 600 °F), sometimes up to 482 °C (900 °F) depending on the cement phases. The two windows stay separate here.
Why Dense Low-Cement Linings Are the Hardest to Dry#
Dense low-cement castables carry the least water but have the lowest permeability after air curing, so their steam has the hardest path out (Jenkins et al., 2011).
The same table ranks ultra-light insulating castables as the least problematic because they are the most permeable, so schedule choice follows permeability as well as water. Cement levels and binders are compared under low-cement, ultra-low-cement and no-cement castables.
Castable Curing Before the First Heat#
A castable cures for at least 24 hours at about 21 to 32 °C (70 to 90 °F) before dry-out starts, and never enters dry-out before a hard set (HWI, 2020; HWI Schedule 2 PLUS, 2013). Below 16 °C (60 °F) the same producer guidance extends the cure or adds heat, and an uncured castable is never allowed to freeze.
Cold curing changes the hydrates: curing below 10 °C (50 °F) forms CAH10 and alumina gel and raises the risk of explosive spalling, according to an Almatis producer paper. Boiler practice keeps refractory wet or humid for 24 h before heating (Bases, Insulation Outlook, 2010).
These 24 h figures are the cited documents' values for their own products, not a rule for every castable. Bagged material sold as refractory cement follows its own sheet, and which product the trade term means is explained under refractory cement.
Refractory Dry-Out Procedure Step by Step#
A refractory dry-out procedure has eight steps, from confirming the producer schedule and the cure to recording temperatures through cool-down, and each step answers to the written plan the owner approved. Values in this section come from HWI Schedule 2 PLUS (2013-12-17) unless another source is named. The eight steps are listed below.
- Confirm the producer schedule and the approved plan.
- Check that the castable has reached a hard set.
- Remove wood forms.
- Provide weep holes or wicking where the cold face has no permeable path.
- Place thermocouples about 13 mm (1/2 in) from the lining.
- Run ramps and holds on the hot-gas temperature.
- Hold or reduce temperature on heavy steam until it subsides.
- Cool down at the schedule rate and record every temperature.
Thermocouples and the Hot-Gas Reference#
Dry-out schedule temperatures are hot-gas temperatures in contact with the lining, not lining temperatures (Schedule 2 PLUS; Hotwork, 2002). The sheet places control thermocouples about 13 mm (1/2 in) from the lining surface.
Installation errors reach 50 °C (a 90 °F difference) (Hotwork, 2002), so positions are checked and logged. Those logs belong to refractory inspection and quality control, and as-installed checks are covered under refractory inspection and quality control.
Ramp Rates, Holds and the Thickness Rule#
Producer and trade schedules ramp at 28 to 56 °C/h (50 to 100 °F/h) and hold for 1 h per 25 mm (1 in) of lining thickness (Schedule 2 PLUS; Bases, 2010). Schedule 2 PLUS holds once, at 649 °C (1,200 °F); Bases holds at three bands, the first at 104 to 204 °C (220 to 400 °F) for cement-bonded material.
Bases counts the thickest section to be dried, while Schedule 2 PLUS counts the total thickness of a multi-layer lining. Worked example: a 229 mm (9 in) single-component lining holds 9 h at 649 °C. Heavy steam calls for a hold or a temperature reduction.
Straight-Line Ramps Versus Ramp-and-Hold Schedules#
A continuous ramp of 7 °C/h (12.5 °F/h) gives a lower peak internal steam pressure than 14 °C/h (25 °F/h) ramps with holds over the same total time, the dry-out philosophy Jenkins et al. (2011) attribute to Hotwork. The lower peak comes from even steam release, not from a longer dry-out.
Cool-Down and Later Heat-Ups#
Schedule 2 PLUS cools from 649 °C (1,200 °F) at 56 °C/h (100 °F/h) and reheats a dried lining at the same rate. Layer thickness comes from refractory lining design, covered under refractory lining design.
Brick-lined vessels start up and shut down after the initial dry-out at no more than 56 to 69 °C/h (100 to 125 °F/h) (Thorpe, cited by Jenkins et al., 2011); faster cool-down loosens the rings.
Example Dry-Out Schedules Compared#
Published dry-out schedules differ in cure, ramp and hold temperatures, and the four in Table 3 show the range: a castable producer sheet, a boiler practice article, a steel ladle practice and a brick-lined vessel rule. On Schedule 2 PLUS a 229 mm (9 in) lining needs about 25 h of dry-out, about 49 h with the cure. Every row of Table 3 is producer or trade guidance.
| Schedule (source, date) | Applies to | Cure | Ramp | Holds | Cool-down and reheat |
|---|---|---|---|---|---|
| HWI Schedule 2 PLUS (HWI, 2013-12-17) | castables and gunning mixes rated for it, one-sided dry-out, single-component linings up to 229 mm (9 in) | at least 24 h at 21 to 38 °C (70 to 100 °F); hard set first | 56 °C/h (100 °F/h) to service | 649 °C (1,200 °F), 1 h per 25 mm (1 in) | cool 56 °C/h (100 °F/h); reheat 56 °C/h (100 °F/h) |
| Boiler practice (Bases, Insulation Outlook, 2010-03-01) | steam-generating boilers | 24 h wet or humid | 28 to 56 °C/h (50 to 100 °F/h); example 42 °C/h (75 °F/h) | 104 to 204, 288 to 371, 482 to 593 °C (220 to 400, 550 to 700, 900 to 1,100 °F); 1 h per 25 mm (1 in) of the thickest section | see product sheet |
| Steel ladle castables (Nippon Steel Technical Report No. 125, 2020) | steel ladle refractory lining | see product sheet | slow at 300 °C (572 °F) and below | above 300 °C (572 °F) to remove residual moisture; 100 t ladle: gas burner 24 h vs hot air at 250 °C (482 °F) plus microwave 8 h | see product sheet |
| Brick-lined vessels after the initial dry-out (Thorpe, cited by Jenkins et al., 2011) | brick rings in shells | n/a | not more than 56 to 69 °C/h (100 to 125 °F/h) | n/a | same limit for shutdown |
Boiler Refractory Dry-Out Schedule#
Boiler refractory is dried after a 24 h wet or humid cure, with ramps of 28 to 56 °C/h (50 to 100 °F/h) and three holds (Bases, Insulation Outlook, 2010-03-01). The holds sit at 104 to 204 °C, 288 to 371 °C and 482 to 593 °C, each 1 h per 25 mm (1 in) of the thickest section.
The example ramp is 42 °C/h (75 °F/h). This is trade guidance, and the installed product's schedule applies where the two differ. Boiler refractory varies by zone, covered under boiler refractory.
Fired Heater and Sulfur Recovery Unit Dry-Outs#
Fired heater and sulfur recovery unit dry-outs are governed by the water in their castable layers, which can sit behind low-permeability brick (Jenkins et al., 2011). In one lining, 229 mm (9 in) of 90 % alumina brick covered 76 mm (3 in) of dense and 76 mm (3 in) of insulating castable holding about 9.5 m³ (2,500 US gal) of water.
In one planned case, 12 h holds at 121, 260, 538, 816 and 1316 °C (250, 500, 1,000, 1,500 and 2,400 °F) could not be delivered by the main burner, the case for third-party burners.
Refinery and petrochemical refractories differ by unit, and heater, reformer and SRU linings are covered under refinery and petrochemical refractories.
Dry-Out by Binder System: Cement, No-Cement, Colloidal Silica and Phosphate#
The binder decides when a castable's water leaves: cement hydrates from 100 to 550 °C (212 to 1,022 °F), colloidal silica below 165 °C (329 °F), and phosphate bonds turn insoluble only at 300 to 350 °C (572 to 662 °F). Table 4 compares the binder systems and the drying additive.
| Binder system | Water leaves or bond forms at | What the dry-out must respect | Source type |
|---|---|---|---|
| Calcium aluminate cement (conventional and low-cement castables) | hydrates from 100 to 130 °C (212 to 266 °F) up to 530 to 550 °C (986 to 1,022 °F) | cure warm; cold curing forms CAH10 and gel | thermal analysis paper; Almatis producer paper |
| Microsilica gel, no-cement (microsilica in refractory castables) | hold 220 °C (428 °F) for 10 h worked; 160 °C (320 °F) for 6 h failed | low-temperature hold before rapid heating | Elkem block study |
| Colloidal silica, no-cement (colloidal silica binders) | nearly all water below 165 °C (329 °F), one stage | keep the sol from freezing | lab result, Salomao et al. 2025; Almatis producer review |
| Phosphate (phosphate binders) | strength from about 100 °C (212 °F); insoluble bond at 300 to 350 °C (572 to 662 °F) | reach the cure temperature | producer guidance |
| Organic drying fibers (additive, any binder) | polyethylene melts at 100 to 120 °C (212 to 248 °F); polypropylene above 150 °C (302 °F) | fibers open steam paths once melted | peer-reviewed paper, Bezerra et al. 2020 |
Calcium Aluminate Cement Hydrates and Their Release Temperatures#
Calcium aluminate cement forms several hydrates plus alumina gel, and each loses its water in its own temperature band in thermal analysis. The bands run from CAH10 and gel at 100 to 130 °C (212 to 266 °F) to C3AH6 at 300 to 360 °C (572 to 680 °F) with a further step near 450 °C (842 °F).
Boehmite dehydrates last, at 530 to 550 °C (986 to 1,022 °F), so cement-bonded linings release water above the 200 to 320 °C window. Cold curing favors CAH10 and gel; grades are covered under calcium aluminate cement for refractories.
Microsilica-Gel and Colloidal Silica No-Cement Castables#
No-cement castables bound by microsilica gel or colloidal silica dry differently from cement-bonded ones and from each other. In an Elkem study on 800 x 600 x 200 mm (31.5 x 23.6 x 7.9 in) microsilica-gel blocks, a 220 °C (428 °F) hold of 10 h before rapid heating worked; a 160 °C (320 °F) hold of 6 h failed.
A colloidal silica bonded castable lost nearly all its water below 165 °C (329 °F) in one stage, and after 120 to 350 °C (248 to 662 °F) its permeability was up to two orders of magnitude above a calcium aluminate cement castable (lab result, Salomao et al., UNITECR 2025 conference paper). Freezing destroys the sol, according to an Almatis producer review.
Organic drying fibers melt in the first heat-up, polyethylene at 100 to 120 °C (212 to 248 °F) and polypropylene above 150 °C (302 °F), and open steam paths (Bezerra et al., Ceramics International, 2020). Fiber types are covered under organic fibers and drying agents for castables.
Phosphate-Bonded Linings#
Phosphate-bonded plastics and mortars gain high strength from about 100 °C (212 °F), and their bond becomes insoluble only at 300 to 350 °C (572 to 662 °F), the recommended cure in the binder producer's guidance. Until then water can still soften the bond.
Common Dry-Out Errors and What They Cause#
Published dry-out failures trace back to seven errors: heating before a hard set, frozen or cold-cured castable, wood forms left in place, misplaced thermocouples, no steam escape path, an oversized main burner, and counting one layer only. The seven errors are listed below with their documented values.
- Heating before a hard set: Schedule 2 PLUS forbids it.
- Frozen or cold-cured castable: curing below 10 °C (50 °F) forms CAH10 and gel.
- Wood forms left in place: they ignite around 230 °C (446 °F), occasionally over 500 °C (932 °F) locally (Hotwork, 2002).
- Misplaced thermocouples: errors reach 50 °C (90 °F difference).
- No steam escape path: weep holes or wicking are required.
- Oversized main burner: in one case about 633 °C/h (1,140 °F/h); the castable separated 25 to 51 mm (1 to 2 in) from the tube sheet (Jenkins et al., 2011).
- Counting one layer only: the hold follows total multi-layer thickness.
Contractor quote scopes name the shared risk: trapped moisture under rapid heating causes cracking, spalling and failure. A cold repair needs a local dry-out of the re-cast area, covered under refractory repair and maintenance.
Refractory Dry-Out Cost: Quote Factors#
Refractory dry-out is priced per quote, because no public price list for dry-out exists (checked 2026-10-27), and six factors set the price. The six quote factors are listed below.
- Burner hire, including third-party burners.
- Fuel over ramps, holds and cool-down.
- Thermocouples and recording.
- Duration in days.
- Cold-weather enclosure below 16 °C (60 °F).
- Access and the outage window.
Quote scopes list dry-out and heat-up as their own block inside the installation scope, run by a specialist where needed.
Contractor Match for Dry-Out and Heat-Up Work#
A dry-out request names the lining, its thickness and layers, the castable products and their producer schedules, the heat source available and the outage window. Those items size burners, fuel and days.
How Does Dry-Out Connect to Installation, Inspection and Repair?#
Dry-out is one step of a reline: the lining is designed, anchored and installed before it, and inspected and, when needed, repaired after it. The related pages are listed below in reline order.
- refractory anchors: anchors fixed before casting.
- refractory installation methods: casting and gunning.
- refractory castable quantity calculator: material per lining volume.
- refractory calculators and quote tools: all tools.
- monolithic refractories: the family hub.
Do Fired Brick Linings Need a Dry-Out?#
Fired bricks hold no mixing water and need no curing, but a brick lining still needs a controlled heat-up, and any castable or mortar behind it sets the dry-out. Refractory bricks heat up per producer guidance; classes are compared under refractory bricks.
Brick rings work by interference fit, so start-up and shutdown stay within the Table 3 limit. In the sulfur recovery lining above, the castable behind the brick governed the dry-out.
Do Insulating Castables Need a Different Dry-Out?#
Insulating castables carry far more water than dense ones, 20 to 40 % by the Jenkins family table and 47 % in the Firelite 2600LI G data sheet (Morgan Thermal Ceramics), but they are the most permeable after air curing.
The Kaolite 2000LI Gun AHR data sheet asks for a 24 h damp cure, 16 to 27 °C (60 to 80 °F) in cold weather and a slow first heat; lightweight grades are covered under insulating castables.