Blackford [5], in her review of ice, defined sintering as a thermal treatment for bonding particles into a coherent, predominantly solid structure via mass transport events that often occur on the atomic scale and Paterson [6] said that sintering of ice and snow is driven by the thermodynamical requirement to decrease surface energy, inducing an increase in both density and strength. The initial stages involve the transformation of the snow ice crystals into a spherical form. Because the radius of an ice particle is larger than the concave radius of the bond joining it to an adjacent ice particle, there is a driving force to move mass to this concave neck area. Two previously spherical particles start tending towards a dumb-bell shaped particle joined by a neck; this bonding leads to improved strength and a lower energy system [7].

This process of mass distribution occurs throughout the sintering process. When this process occurs under additional pressure such as when snow is buried owing to accumulation, it is termed pressure sintering, and the rate of sintering is increased. Material densification results from the sintering process; however, Alley [8] clarifies that although numerous processes occur throughout the pressure sintering process, not all contribute to densification. Herron and Langway [9] identified three main phases in the densification process: settling and packing of particles, pore-space reduction, then pore-space closure. In snow, the densification process slows down in each stage, and also occurs intermittently throughout the year, depending on accumulation rates [10]. This process of intermittent densification was recently demonstrated by Einav and Guillard [11] using rice crispies and was previously described as self-organised criticality by Sammonds [12].

The primary mechanical observation to be drawn from this discussion is that the strength of snow is controlled by the size of the bonds formed between grains through sintering [13] and that the rate and amount of sintering occurs at different rates under varying environmental conditions.

The increase in snow strength attained over time can be assessed in various ways [14]. One of the most efficient means is penetration testing. Schneebeli [15] has extensively examined the strength of snow undergoing penetration with a micro-penetrometer and McCallum [16] has examined such behaviour using a friction-sleeve-equipped cone penetrometer. More recently, Peinke et al. [17] examined snow sintering at microscopic and macroscopic scales with high-resolution cone penetration tests. They showed that macroscopic penetration force increased over time due to microstructural bond strengthening.

In the absence of snow, other analogues have served as a proxy to describe the behaviour of snow under mechanical testing; these include:

• Sintered metal. Snow can be considered as a sintered material such as a metal, where, under stress, the usual fracture of the sintered necks takes place, as for most porous material [18]. This behaviour is consistent with the failure mechanism for snow described by Mellor [19].
• Geomaterials. Leroueil and Vaughan [20] showed that deposits normally treated as soils usually have characteristics due to bonded structure which are similar to those of porous weak rock, resulting in mechanically stiff behaviour followed by yield. They showed that such characteristics are common in natural geological materials and that it is the structure of weak rock and cemented sands that gives them their strength. This structure, arising from different causes, gives similar behaviour in many different materials including snow [20].
• Sedimentary material. Snow has historically been viewed specifically as a sedimentary material (Benson [21], Pielmeier and Schneebeli [22] and Schweizer et al. [23] amongst others) that ultimately may develop structure and thus behave in accordance with Leroueil and Vaughan’s supposition [20] above.
• Porous Rock. Leite and Ferland [24], in their work on the indentation of porous material, note linear elasticity, yielding and structural collapse at a critical value, and strain hardening as the crushed material is compacted; all these behaviours are observed in the penetration of snow (of certain density). Tharp [25], in his work on polyphase rocks, suggests that incompetent phases (essentially non-load-bearing phases such as air) within a material result in the load-carrying framework behaving much like a porous solid, and snow has been considered as such by numerous authors including Brown [1], Kirchner et al. [4], and Petrovic [2].
• Foam. Snow has been specifically considered as a foam or a cellular solid by Brown [1], Kirchner et al. [4], Petrovic [2] and others including Johnson [3]. In his work on a statistical micromechanical theory for penetration in granular materials, Johnson [3] draws upon initial work presented by Gibson and Ashby [26], who examine the behaviour of foam under penetration in their review of the behaviour of cellular solids. Examination of Gibson and Ashby’s work [26] suggests that below the pore close-off density of ∼840 kg m−3, snow might be regarded as an open-cellular foam, whilst above this density, description as a closed-cellular foam may be appropriate.
• Polycrystalline ice. The main constituent of dry snow is ice, thus snow’s behaviour must tend towards that of polycrystalline ice as density increases. Nicot [27] states that on the microscopic scale, the behaviour of grain bonds (within snow) is governed by the behaviour of ice, Gubler [28] describes the load-bearing capacity of snow in terms of ice chains, and Bartelt and von Moos [29] note the straining of the ice lattice within snow during triaxial testing. Graphs of stress versus strain rate generated by Kinosita [30] through loading of snow are almost identical in form to those presented by Schulson [31] for polycrystalline ice.

Sugar is a synthesised organic material available in several shapes and sizes [32]. Raw sugars are refined and softened by removing the molasses coating through the process of affination [33]. The resulting syrup is then processed by precipitating and filtering solids in the syrup, then the liquor is spun using a centrifuge, separating the white crystals from the solution. These white crystals are dried in hot air and can then be used or packaged [34].

When stored in a sealed container, bonding between sugar grains is not evident [35]. However, when stored in a high humidity environment, sugar crystals bond; Bagster [36] speculated that these intergranular bonds form via crystalline bridging. Bagster [35] demonstrated an increase in raw sugar shear strength over time (out to ∼100 days), in a constant humidity environment. Zafar et al. [37] showed that sintering in a ’powder’ such as sugar is influenced by time, temperature and moisture, and that increased bonding will result in an increase in material strength, similar to what is observed in dry snow.