Salamander cooling season: gelling behaviour at cellar temperatures

Every November the same call comes in. Someone has mixed a batch in a cold room, judged it too dry, added more powder, and ended up with a floor that set like damp concrete by morning. The batch was fine. The water was 9 C, and polymer at 9 C takes roughly three times as long to swell as it does at 20 C.

Cooling season changes the chemistry of the job more than most keepers expect. Salamanders and newts want a cool floor, often 10 to 14 C, and that is exactly the range where gel behaves differently, sets slower, and holds its shape longer once it has set. Get the mixing right and the cold works in your favour. Get it wrong and you will be chipping a solid layer out of a glass tank with a plastic scraper.

Cold water hydrates slower than you think

Polymer granules absorb by drawing water into a cross-linked network. The rate depends on viscosity and diffusion, and both slow down as temperature falls. A dose that reaches full swell in 25 minutes at 22 C may need 80 minutes at 10 C. If you mix on a schedule learned in summer, you will see a slushy, half-swollen batch at the moment you expect a firm gel, and the temptation is to add powder.

Adding powder to a cold half-swollen batch is the single most common way to ruin a winter mix. The new granules compete for the remaining free water, the batch stiffens, and you end up with unswollen cores inside a stiff matrix. Those cores keep absorbing for days, which is why the floor gets harder after you have already put the animal in.

What 12 C does to a set mix

Once hydrated, a cold substrate behaves well. Evaporation drops sharply, so a floor that would need a light mist every second day in July can go a week in a cellar. Gel releases water more slowly at low temperature too, which means the moisture sits in the polymer instead of moving into the surrounding fibre. The result is a floor that feels dry on top and stays wet underneath for longer than you would guess.

That has two consequences. First, you can run a lower gel share in winter without the floor drying out, which saves money and reduces the springy texture that salamanders dislike. Second, the top layer can mislead you into misting a floor that does not need it. Push a finger down 3 cm before you pick up the sprayer.

Mould down, anaerobic risk up

Cold rooms slow fungal growth, and a lot of keepers relax because of it. The risk that replaces mould is anaerobic. A cold, wet, tightly packed floor with little air movement can go sour at the bottom while the surface looks and smells fine. You notice it when the animal stops burrowing, or when you break the floor down at the end of the season and find a grey layer with a sulphurous smell at the drainage interface.

The fix is structural rather than chemical. Keep the bottom 2 cm of the floor open with a coarse layer, do not pack the lower half, and let the surface dry between waterings. A floor that is 12 cm deep with a 2.5 percent gel share and a loose base will stay aerobic for months. The same floor packed tight at 5 percent will not.

The winter mix I would use

Mix with water at room temperature and let the batch stand somewhere warm before it goes into the tank. A batch hydrated at 20 C and then cooled holds far more usable water than one hydrated at 10 C, because full swell happened before the cold. This one change removed most of the winter complaints we used to get from keepers running cellar racks.

For the temperature side of the same problem, the notes on exotic pet humidity control are worth a read before the season turns.

Test your first cold batch in a clear tub before it goes near an animal. If you can press a thumb into it and the dent stays, the swell is complete. If the dent fills back in, it is still working. Wait, then place it.