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April 17, 2020 /
Wheels of Steel
The pros and cons of stainless steel, copper, and brass when it comes to boilers, portafilters and blocks.

Call me a nerd, but I vividly remember the first time I used a machine made with all stainless steel. The coffee tasted vibrant and clean, and the salesperson showed me how easy it was to wipe away the residue from inside the portafilter — no more scrubbing away with a Scotch-Brite at the end of every day. The steel was a big selling point: easy to clean, more environmentally friendly, lead-free, resistant to limescale. It seems like a no-brainer, and indeed, we went on to buy the machine.

Since then, the number of machines being made with steel boilers, groups, or portafilters seems to have increased — but most are still made the traditional way, with copper boilers and pipes, and brass fittings, groups, and portafilters.

Brass and copper have both been linked with lead contamination in drinking water, leading to ever stricter legislation on how they are used. With increasing pressure on manufacturers to reduce their use of these materials, there must be some pretty strong reasons to continue using them instead of steel.

In this post, we’ll look at why copper and brass are so useful in espresso machines, where steel has the advantage, and whether there’s any reason to be worried about the amount of lead in our coffee.

 

Thermal properties

One of the major reasons that copper has historically been used in boilers and cookware is that it’s such a good conductor of heat. Copper has a thermal conductivity of over 400 W/mK, compared to stainless steel at about 14 W/mK.

The conductivity of brass varies, depending on how much copper it contains, but typically lies around 100 14 W/mK, but has the advantage over copper of being much harder and stronger.

This means a copper boiler or pan is much more effective at transmitting heat from the flame underneath, to the water inside it, than a steel one.

Inside an espresso machine, these properties can be important in unexpected ways. Even inside a boiler, where you’d think that the insulating properties of steel would be an advantage, it can create problems. Michael Teahan, director of Analogue Coffee, says that when he was working on superautomatic machines at Brasilia, they found that the steel steam boiler was so inefficient at conducting heat to the top of the boiler, steam would condense on the upper surfaces, creating wet steam. To solve this problem they had to reheat the steam en route to the steam valve. “Stainless boilers on traditional machines are usually smaller in diameter to minimise the problem of heat transfer,” he says. “The problem with steel is that it sucks at moving heat around.”

Water itself is a poor conductor of heat, and steam even more so; so to transfer heat from the boiler to the group, you either need to use a more conductive material, like copper or brass, or rely on the movement of water to carry the heat around. 

In a heat exchanger machine that uses a thermosyphon to transfer heat to the grouphead, it’s the movement of water that carries the heat from the boiler to the group. But even here, copper and brass have an advantage: the movement of water in the thermosyphon relies on the water cooling at the group. This makes the cooler water sink and return to the heat exchanger, creating a constant flow of water through the group head. “For heat to migrate, the group head — including the portafilter — has to radiate heat,” Teahan explains. This is much more effective with a conductive material like brass.

Machines based on steel often need to use different methods to transfer heat. For example, La Marzocco, who started using stainless steel boilers back in 1971, use the ‘saturated group’ design, where a chamber in the group is filled with water and continuous with the boiler. This design uses the circulation of water throughout the system to transfer heat to the group, explains Enrico Wurm, Product Improvement Manager at La Marzocco. “It is true that steel retains less heat than brass, however we compensate for this problem by building heavy and thick groups to get more thermic inertia, and by [insulating] our boilers.”

Other modern espresso machine designs get around this problem by heating the group in different ways: for example by heating the material of the group directly, such as in Nuova Simonelli’s Aurelia II T3, or even by placing the brew boiler directly above the grouphead like in Sanremo’s Opera. This means such machines rely less on the material to conduct heat, making steel a more practical possibility.

Left: San Remo Opera, right: Nuova Simonelli Aurelia II T3.

 

Mechanical Properties

Copper is relatively soft, which makes it easy to work and draw into pipes. The smaller pipes in espresso machines can even be bent into shape by hand, which together with its conductivity, means it’s still used for pipework in many otherwise all-steel machines.

In some places, especially in domestic and superautomatic machines, copper is replaced by PTFE (Teflon) tubing, which is flexible and cheap. However it can only be used in situations where the conductivity and rigidity of copper isn’t needed.

As brass contains copper, it retains some of these properties. It has a low melting point, which makes it easy to cast, and is relatively soft and has low friction, which makes it easy to machine. This makes it ideal for threaded parts or more intricate components, especially where moving parts are involved — so machines with steel boilers and groups might still use brass for safety valves, or compression nuts to connect pipework.

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Steel is much stronger, but harder to work than brass or copper, and more brittle. This is another reason that steel tends to be used for smaller boilers, which are at less risk of cracking. Manufacturers that choose to work with steel have to switch to different manufacturing techniques, explains Wurm. “316L [steel] is a very hard alloy, difficult to machine with normal tooling equipment, and also very difficult to weld,” he says. “This material forced us to master techniques such as TIG and plasma welding. Today our groups are made through the lost wax casting technique, which means that they are made of 100% solid steel, with no junction or welding.”

Because brass is easier to machine and cast, it ends up being cheaper to work with, despite the higher cost of the raw materials. However, these properties of brass rely on the alloy containing a percentage of lead (N Gane, 1981). Without lead, this advantage disappears — stainless steel can actually be easier to machine than lead-free brass, according to Teahan, and lead-free brass can break moulds used in traditional casting methods. However, working with steel is still more expensive overall, claims Wurm, due to the skills and labour needed to build such machines.

 

Lead

The lead content that gives brass its useful properties has also become one of the big drivers away from using it in espresso machines. Lead in drinking water has been linked to numerous health problems, especially in children, where even small amounts can result in developmental problems, or behaviour and learning issues. As a result, there is considered to be no safe level for lead in drinking water (US EPA).

This is one of the reasons that La Marzocco moved to using steel in groupheads as well as boilers. “Water quality all over the world has gotten worse in the last 10 years,” Wurm says. “Therefore we transitioned to materials that could better withstand [corrosion] and thus release no heavy metals.”

Legislation around the use of lead has also become increasingly strict. The parts of a machine in contact with water in the US must now contain no more than 0.25% lead, by total weight. In Europe a similar standard applies, with 0.3% lead permissible.

To comply with legislation, some manufacturers opted to coat brass surfaces to prevent contact with water, for example using a process called Ternary Eco Alloy (T.E.A.). Others switched to so-called ‘lead-free’ brass, with less than 0.25% lead content. In ‘lead-free’ brass, other additives such as Bismuth or Silicon can be used to replace lead, to make the brass easier to machine (J Choucri et al, 2019).

However, since ‘lead-free’ brass can still contain up to 0.25% lead, some lead can still be released into the water. If the water is stagnant, the amount of lead released can become enough to breach guidelines for safe drinking water (D-Q Ng & Y-P Lin, 2016).

So should consumers be concerned about being exposed to lead from espresso machines?

“Honestly, no,” says Teahan. “The main problems with lead are with child development,” he points out. “Once you get to the age you’re drinking coffee, you’re pretty much set.”

While there have been scare stories about lead found in coffee from cafes, more systematic research has shown that lead exposure from coffee is fairly low (Danish EPA, 2015). What lead there is in coffee seems to come from the coffee itself, rather than from the equipment, according to the study. “Regardless of the brewing method used, all the lead present in the coffee beans was extracted [into the] brewed coffee… There was no indication that lead was extracted from home brewing equipment.”

The researchers didn’t test espresso machines specifically, but found that coffees from cafes in Denmark had similar lead content to coffees from home brewers. “The intake of lead from coffee is low compared to the intake from other dietary sources, and it does not constitute a major part of the total dietary intake of lead,” they conclude.

Other sources of lead are likely to be more important, and the main source of lead exposure for children in the US is house dust (US CDC, 2017). In fact, even just the choice of the cup you drink your coffee from can be enough to cause lead exposure to exceed the California Maximum Allowable Dose Level (GL Anderson et al, 2017).

However, home baristas do need to be a little more careful. A government research project in Germany found high levels of lead were released from home espresso machines, especially after descaling (BfR, 2013). Since home machines are used less often, the water is also in contact with lead-containing parts for much longer, which can increase the amount of lead leached into the water. To limit exposure, the researchers recommend flushing machines before use, and rinsing machines thoroughly after any descaling process.

 

What about Aluminium?

Aluminium is fairly cheap, light, strong, and a good thermal conductor — in many ways an ideal material for boilers. However, it is toxic if released into the water (C Exley, 2016), so where it is used, it is often coated or lined with steel. Aluminium is typically only used for small boilers in domestic machines, or for thermoblocks in superautomatic machines, where the conductivity is especially useful. However, it can’t be used anywhere that it would come into contact with acidic coffee, which could cause aluminium ions to be released.

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