Salox - Sound Science - Good Sense
Engineering metal for the mouth - a warm and soft environment
Metals are the first choice for modern horse bits. They have the basic strength to meet the demands of a powerful sport and recreation such as riding and can be formed into an infinite range of precisely defined shapes.
But other than these basic requirements, what other properties would the perfect metal possess?
Neue Schule's answer to this question is based on a simple philosophy – that which is good for the horse is all that matters – the rest will follow.
The mouth is warm and soft and needs to remain as calm and still as possible to listen for the rider’s aids. So, after ensuring that basic engineering strength and toughness is satisfied, the metal should then also be:
Let’s take these three requirements from the pure science perspective:
– meaning, in general, ‘comfort’
‘Hot’ and ‘cold’ are sensations for the horse that depend on the rate at which heat energy is transferred from, or to, an object in contact with it. This rate depends on the temperature difference between the surface of the object and the surfaces of the horse’s mouth as well as the thermal diffusivities of both structures. In physics, the horse, being far more massive and warmer than the bit, is called the ‘thermal bath’ and the bit will heat so that it takes up the temperature of this warm ‘bath’. In this case we can consider the thermal diffusivity of the bit material alone and compare between different materials.
In materials with very low diffusivity, such as polymers, the temperature difference at the contact interface becomes very small a short time after first contact. However, the bit will not fully reach a uniform temperature throughout its volume for a long time. This can feel comfortable because of the small temperature difference at the interface and the very slow rate at which heat energy is transferred from the horse to the center of the bit.
Conversely, in materials with very high thermal diffusivity, such as copper, the rate of heat energy transfer is very fast and a high temperature difference between the bit and the horse lasts for the short time it takes for full heat transfer from the horse to the bit. Throughout this the bit will feel ‘cold’ before it rapidly acquires the temperature of the ‘bath’ and will then feel comfortable. The aim is to make this heat transfer time as short as possible.
In between these extremes, variable degrees of discomfort are possible for longer times because the heat transfer rate is intermediate and the large temperature difference and moderate rate of heat transfer maintains the ‘cold’ feeling for longer. Salox comprises copper for high thermal diffusivity, but with necessary strengthening and anti-tarnishing agents. The ideal target would aim to meet a thermal diffusivity level that allows the bit to thermally equilibrate on a time scale comparable to the reaction time of the central nervous system of the horse.
The following table and graph gives Neue Schule experimental values for thermal diffusivity comparing Salox and a typical ‘standard’ copper alloy (SCA) bit material and gives literature values typical for stainless steel and for a high quality engineering polymer that may also be used for horse bits.
Table 1: Thermal diffusivity values (mm2 s-1) measured (SCA and Salox) or taken from literature sources for various horse bit mouthpiece materials
Figure 1: Equilibrium uniform temperature versus normalized time of materials of identical volume and surface dimensions when placed in contact with a thermal bath at 37 °C. The curves are calculated as inverted Newtonian cooling curves with the diffusivities from Table 1 entered for the exponential rate constant
These results show that Salox and the polymer lie at opposite extremes of the scale where comfort is most likely to be achieved. Other materials lie in between these extremes.
There is a possibility that many horses will make variable degrees of contact between the teeth and the bit. When this happens we would like to ensure that the impact forces are safely and comfortably absorbed into the material of the bit rather than the teeth. Not only is this a better experience for the horse but it will reduce tooth wear that can occur with prolonged contact with a hard bit. The material has to be softer than enamel for this and in fact all modern bits use materials with hardness values lower than that of tooth enamel. But again we have a spectrum of values.
Table 2: Hardness values (Vickers 10kgf scale) measured (Stainless steel, SCA and Salox) or taken from literature sources for tooth enamel2 and polymer3
Polymer bits lie at the most yielding end of this spectrum and absorb the impact by either elastic deformation (low tooth pressure) or varying degrees of permanent ‘damage’ (moderate to high pressure) usually by forming small cuts in the surface (exceeding the so-called “tearing” force for the polymer). This property is comfortable for the horse and safe, provided there is not too much removal of material, which needs to be monitored carefully when using these products.
At the top end of the hardness spectrum, stainless steel is quite unyielding and will provide a significant elastic reaction force against the tooth, perhaps best described as ‘jarring’.
So, in Salox we make sure that there is a way for the force to dissipate into permanent deformations (depressions or ‘dents’) in the surface. Unlike polymer bits, repeated contact with the teeth removes little or no material and in fact leads to a slightly more resistant surface over time due to strain hardening. We therefore make Salox strong and extremely tough, but to take advantage of the favorable features of polymer, somewhat softer than all current bit metals.
So far then, we can say that Salox is ‘warmer’ and ‘softer’ than all other bit materials, except for polymers.
Provided the bit is comfortable there should now be nothing that interests the horse about the bit that can become a distraction to the rider’s aids. This includes tastes and smells which are the remaining sensory functions relevant in an object intended for the mouth.
It is known that divalent metal oxides, particularly of copper and iron can excite such sensory responses. Groundbreaking research in 20061 showed that these divalent metal ions in contact with epithelial cells stimulate the production of volatile carbonyl hydrocarbons which account for the odor of metals and odor is an essential part of the experience of taste. These volatile organic compounds are essentially ‘body odors’, but whether they are favorable or unfavorable to the horse they may produce an over-activity in the mouth which is unwanted.
Stainless steel is the perfect example to illustrate how such stimulants can be suppressed. The insoluble protective layer of chromium dioxide on the surface of stainless steel locks in alloy metals such as iron and nickel whose oxides would otherwise become bio-available. This also explains why no nickel allergy is expected from stainless steel.
Following this idea, Neue Schule adds aluminum to the mouthpiece metal composition. It is known that aluminum does not produce the volatile compounds but it produces a protective oxide (alumina, Al2O3) that in part prevents the bit surface from becoming rich in oxides of copper and zinc. A study comparing Salox with an aluminum-free SCA (with closest equivalent standard composition, UNS C69400) strikingly emphasizes the relative reduction in extractable copper and zinc due to this effect.
Thin discs (12.5 mm diameter, 1 mm thickness) were cut from a Neue Schule Salox bit and from an alternative standard copper alloy mouthpiece. The discs were first solvent cleaned and pre-treated in oxygen plasma to ensure full oxidation of the surface. Aqueous extracts were measured by ICP emission spectroscopy after 6 days reflux into boiling phosphate buffer solution at pH7.
Based on the known relative quantities of copper and zinc between the two samples and making no other assumptions the expected extraction quantities should be proportional to the metal content (‘expected’ data). The measured data, shown in figure 2, show a 4 fold relative reduction in zinc and a 2 fold relative reduction in copper as soluble extracts compared to the expected values.
Figure 2: Observed ratio (Salox : SCA) of extracted copper and zinc compared with Expected ratio based on weight percent composition ratio.
Aluminum clearly offers the same method of protection as chromium in stainless steel and gets closer to our goal of inhibiting the availability of copper and zinc ions in contact with the mouth. This will reduce the production of the organic compounds responsible for taste and smell.
This completes the explanation of the science considered to provide what is intended to be a metal that is comfortable, safe and ‘neutral’ to the horse. The aim is to achieve a quiet mouth, ready to receive instructions through the reins.
1. D. Glindemann et al., Angew. Chem. Int. Ed., 2006, 45, 7006 – 7009
2. R.G. Craig and F.A. Peyton, J. Dent. Res., 1958, 37, 661 – 668
3. Value is typical for thermoplastic polyurethane elastomers (e.g. Bayer Desmopan/Texin)