Hot Lights, Four Hours In: Melt, Re-Dry and Continuity

Hour six of a ten-hour day, the floor monitor called for a re-set on the wide because an actor had walked through frame left. The snow in that corner had been under a 5 kW lamp since call time. It was no longer snow. It was a translucent, slightly yellow sheet with a texture like cold porridge, and it photographed as a wet patch on a grey floor.

Hot lights change polymer snow in ways that are easy to predict and hard to fix on the day. Understanding the two-stage failure — first drying, then re-wetting from the base — is what keeps continuity intact across a long shoot.

What actually happens under a lamp

Polymer snow holds water in a cross-linked network. Heat drives that water out of the surface first, which is the stage everybody notices: the drift goes pale, loses sparkle and starts to look like chalk. That is reversible. Mist it, wait ten minutes, and it comes back.

The second stage is not reversible in place. As the surface dries, it forms a skin. The skin insulates the material below, which stays wet and warm. Under sustained heat from above, water migrates upward through the drift by capillary action and evaporates at the skin, and the gel structure at the surface slowly collapses. What is left is a dense, semi-translucent layer with no air in it — hence the porridge look and the yellow cast from the lamp reflecting off a wet surface rather than scattering off flakes.

Time to reach stage two under a 2 kW open-faced lamp at 900 mm: about 3 hours in a room at 22 C, less if the lamp is closer or the drift is shallow.

The continuity method that survives a day

Do not try to keep one drift alive for ten hours. Build the set so that the hero area can be replaced in sections.

• Lay the snow in 600 x 600 mm trays lined with polythene, or mark the floor into sections with tape and build each section separately.
• Keep two spare sections hydrated and covered in a sealed tub at the edge of set. A section swapped in takes four minutes and matches because it was mixed from the same batch.
• Flag the drift during every lighting change, not just at the start. Heat build-up is cumulative and a lamp that is moved 300 mm closer doubles the drying rate.
• Mist at lunch, not at the end of the day. Material that has been dry for six hours needs a full rehydration, not a mist, and a full rehydration on set means a wet floor.

Re-drying and the shrinkage problem

Here is the finding that catches people on multi-day shoots: snow that has dried out and been rehydrated does not come back to the same volume. In bench tests, a batch dried to constant weight and rehydrated at the same water ratio recovers about 82 to 88 percent of its original bulk. The granules fuse slightly during drying, and the re-formed gel is denser.

That means a re-dried batch used for pickups three weeks later will sit visibly lower in the same tray, will photograph darker, and will not blend at the seam with fresh material. It also shrinks more on the second drying cycle, so a batch used on three consecutive weeks is noticeably compacted by the third.

For continuity, keep a sealed reference sample from day one — a litre tub, labelled, unopened — and match every later batch against it by eye and by weight per litre. It is the cheapest continuity tool on the truck.

A re-set that went wrong

On a commercial shot, a crew re-set a corner by lifting the compacted top layer, misting the material below and laying fresh snow on top. The fresh layer sat on a wet base, absorbed water from underneath, slumped overnight, and by morning had a visible ridge where the two layers met. The shot list had a slow push-in across exactly that seam.

The fix was to cut out the whole corner to the tray liner and rebuild. Twenty minutes, but it meant the seam was gone. Patching on top of a wet base never works; the base has to come out.

If the schedule has pickups, plan the trays and the spare sections before the first take. The on-set handling notes cover rehydration volumes for a full re-set versus a light mist.