Gold Leaf Size Tack Time Calculator

How It Works

Gilding size (adhesive) has a characteristic drying curve. It starts wet, becomes tacky at a certain time, remains in its optimal gilding window for a range, then dries too hard for leaf to adhere. Manufacturers publish reference times at 20 °C and 50% relative humidity — the standard workshop condition. Ambient temperature and humidity move that curve forward or backward.

Rolco's Slow Set Size technical data sheet gives 12 hours to first tack and 24 hours to optimal-window close at reference conditions. The Lefranc & Bourgeois Mixtion 3-Hour and Mixtion 12-Hour product data sheets follow the same convention with different numbers. Kolner Instacoll (a waterbased size) reaches tack in as little as 20 minutes because the water evaporates before the acrylic polymer sets. Sepp Leaf Products' Sizing and Gilding guide is the standard reference in the North American gilding trade.

The tool applies a linear adjustment to those published times. A rough rule from Sepp Leaf's guide is that every 5 °C above reference shortens the working window by 10-15%, and every 20% RH above reference lengthens it by 10-15% for oil sizes and a bit less for water sizes. The tool encodes this as factor = (1 − ΔT × tempCoefficient) × (1 + ΔRH × humidityCoefficient), clamped to prevent runaway estimates at extreme conditions.

The result is three timepoints: tack start (earliest leaf), optimal window close (latest reliable leaf), and dry-out (leaf will not adhere at all). The gap between tack start and optimal close is the working window — how long the gilder has to lay leaf. Very short windows (under 15 minutes) mean the size is faster than the work can be completed, so a slower size or smaller area is needed.

Humidity matters more for oil sizes than water sizes because oil sizes cure by oxidation. High humidity slows oxidation and lengthens the entire timeline; very low humidity has the opposite effect. Water sizes cure by evaporation, so low humidity dries them faster while high humidity prevents them from ever fully setting.

How to Use This Calculator

Select your gilding size — Kolner Instacoll, Rolco Slow Set, Lefranc & Bourgeois 3-hour or 12-hour, or a generic acrylic water size.
Enter room temperature and humidity — Measure with a digital hygrometer near the work surface, not just from the wall thermostat.
Read the tack window — The three time markers show when leaf can first be applied, when the optimal window closes, and when the size dries too hard to gild.
Plan your work — Divide the gilding surface into sections you can complete inside the working window. Long surfaces may need a slower size.

Real-World Use Cases

Small picture frame with 3-hour oil size

A 40 × 60 cm oil-gilded frame at 22 °C, 55% RH using Lefranc & Bourgeois Mixtion 3-Hour: the tool shows tack start around 165 minutes (2h 45m), optimal end at about 440 minutes (7h 20m), dry-out at 660 minutes (11h). The working window is roughly 4 hours 35 minutes — enough to lay leaf across the entire frame in one sitting.

Large architectural cornice with 12-hour oil size

A cornice at 28 °C, 40% RH using Rolco Slow Set (published 12h / 24h / 36h at reference): factor is (1 − 8 × 0.025) × (1 + −10 × 0.006) = 0.80 × 0.94 = 0.75. Tack start shifts to about 540 minutes (9 hours), optimal end to 1080 minutes (18 hours), dry-out to 1620 minutes (27 hours). Even in warm dry conditions the working window stays at 9 hours — comfortable for a large surface.

Instacoll gilding on a bookbinding studio

A bookbinder using Kolner Instacoll at 20 °C, 65% RH sees tack start at 24 minutes, optimal end at 288 minutes (4h 48m), dry-out at 1728 minutes (28h 48m). The gilder can lay leaf for close to 5 hours in this condition. High humidity extends the window; in a dry 30% RH winter workshop, the same size shrinks the optimal end to about 168 minutes (2h 48m).

Tips & Safety Notes

Digital hygrometers are cheap and essential. A $15 hygrometer next to the work surface gives numbers a wall thermostat cannot.
Test with a knuckle, not a fingertip. Skin oil on the size prevents leaf from adhering to that spot.
Keep the room stable. Opening a door on a humid or cold day can shift RH by 20 points and derail your timing.
Slow sizes are more forgiving. Beginners should learn on 12-hour oil size; the wider window means small mistakes in timing are easier to recover from.
Cover unsized areas before gilding. Loose leaf will stick to any tacky surface — including fingerprints on adjacent unsized areas.
Store sizes tightly capped. Water sizes can dry out in the container over months; oil sizes can develop skins.

Limitations & What This Tool Cannot Do

Reference times are the manufacturer's own, at their own conditions. Batch-to-batch variation in oil sizes can be ±20% even at identical ambient conditions. The tool provides a first estimate; a small tack test on a scrap in the same room is the real answer.
The adjustment factors are linear approximations. At the extreme ends of the temperature and humidity ranges the tool models (5-35 °C, 20-95% RH), real curves can bend. The tool clamps its adjustment factor between 0.3 and 3.0 to prevent unreasonable estimates.
Substrate absorption is not modeled. Sizing over gesso behaves differently from sizing over sealed metal. The tool provides the ambient-adjusted schedule; the specific surface may still absorb the size faster.
Air movement is not modeled. A fan or draft accelerates dry-out especially for water sizes. Work in still air or accept that the actual window will be shorter than predicted.

FAQ

How do I know when gold size is ready for leaf?

Press the back of a knuckle to the sized surface — correct tack feels sticky but leaves no residue on skin. If it feels wet or transfers, wait longer. If there is no sticky pull, the size has dried past its usable window. Never test with a fingertip because oil from the finger contaminates the surface.

What is the difference between oil size and water size for gilding?

Oil size is a linseed or synthetic oil varnish, longer working window (3-24 hours), gives the highest gloss, and is standard for architectural and fine gilding. Water size is a waterbased acrylic emulsion, short set (15-60 minutes), reactivatable with breath before it fully dries, matte-to-satin finish, and standard for small interior work.

Does temperature really change gold size tack time that much?

Yes. A 10 °C rise typically shortens oil-size working windows by 25-30% and water-size windows by 30-35%. Combined with humidity changes, a size rated for 12 hours at 20 °C / 50% RH can behave like an 8-hour size in a 28 °C summer workshop.

What happens if I apply gold leaf too early or too late?

Too early (before tack): leaf sinks into the wet size, loses its bright metallic surface, and can wrinkle. Too late (after dry-out): the leaf will not adhere at all and blows off, or adheres only partially with visible skips. The optimal window is when the surface pulls the knuckle back slightly without transferring.

Can I use the same size for both interior and exterior gilding?

For exterior work, use a slow-set oil size (12-hour or longer) — the harder cured film resists moisture and UV. Water sizes and acrylic emulsions are not durable outdoors. For interior gilded picture frames, book edges, and architectural details, water size and acrylic sizes are lighter to work with and fully durable.

Related Tools

Varnish Dilution Calculator — Oil-based finishes share solvent-evaporation behavior with oil-based gilding sizes.
Resin Exotherm Calculator — Temperature and cure timing for a related exothermic reaction.
Cyanotype Sensitizer Calculator — Another humidity-sensitive workshop process where ambient conditions drive timing.

References

Kolner Instacoll, product technical data sheet (kolner-online.com — Instacoll TDS) — waterbased size reference times and rewet behavior
Rolco, Slow Set Size technical data sheet (rolcopaint.com) — 12-hour oil size reference times
Lefranc & Bourgeois, Mixtion 3 Hours and Mixtion 12 Hours product data sheets — fast and slow oil size reference times
Sepp Leaf Products, Sizing and Gilding guide — trade-standard adjustment rules for temperature and humidity

Reviewed by the Craft Calc Lab team on August 5, 2026.