A_A , 9 months ago (edited 9 months ago)

Yes a bucket of a mixture of small ice pellets, say a few millimeter size, plus water, (this bucket being enveloped with some insulation) would be a great zero degrees Celsius reference point.

if you want something more precise you can read this :
en.m.wikipedia.org/wiki/Celsius

… “the actual melting point of ice is very slightly (less than a thousandth of a degree) below 0 °C.” …

isotopic distribution of heavy and light elements in water also has a very slight effect on melting point. So, rainwater and water distilled from ocean will not melt at the (exact) same temperature.

See : Vienna Standard Mean Ocean Water …wikipedia.org/…/Vienna_Standard_Mean_Ocean_Water

Now, about small particle fusing together this is true not only of ice but of any material.
it’s called sinteringand it is caused by diffusion and a lowering of the surface energy.
This process is faster when the material is near it’s melting temperature and faster yet if in contact with any miscible liquid phase.