Early heat determination in the brewery

Heat management before thermometry

Our ability to recover early techniques depends on the written record; we must be aware that this was not necessarily representative, especially before the middle of the eighteenth century. The convention of transcribing easily-interpreted accounts was itself an emerging innovation among brewers: publishing was an expensive business, while knowledge of brewing procedures could be a valuable trade secret. Commercial brewery insiders had a strong incentive to preserve the 'mystery' of their craft, inducting new initiates by direct experience and seldom recording instructions on paper. Conversely, outsiders attempting to promote new practices were among those most likely to publish. One such was the autodidact food reformer Thomas Tryon, who claimed that hops should never be brought to boil in the wort; ² this subjected him to the hostility and ridicule of commercial brewers, and the practice was probably never widely taken up. ³

Our best early source, the first dedicated brewery manual to extend beyond pamphlet length, is the London and Country Brewer, published anonymously between 1734 and 1740 by William Ellis, a Hertfordshire gentleman-farmer better-known for his writings on husbandry and related subjects. ⁴ Ellis had at some point gained experience of the London brewery, serving as executor to a brewing uncle's will; ⁵ though some contemporaries rebuked him for uncritically reporting or misunderstanding the claims of others, ⁶ it is probable that some of his brewery information was gathered at first hand.

Ellis describes how an ‘hour-glass’ tradition, whereby mashing liquor was brought to boiling point and then cooled for a fixed period before use, ⁷ gave way in the early eighteenth century to judgment based on the brewer's own senses and experience. ⁸ There is a tendency in historical accounts of brewing to focus on the literal 'rule of thumb,' or tactile criterion - the mashing liquor being heated to the highest degree the brewer's thumb or elbow could bear^{9 & 10} - as this was the practice most often excoriated as oldfashioned by early thermometrists. ¹¹ My research, however, has revealed only one source positively recommending tactile judgment. ¹² Far more commonlydescribed in early sources is the process of heating the water to near boiling, then leaving it to cool until the steam subsides to the point where the brewer can see his own face reflected in the surface.^{13, 14 & 15}

Interestingly, qualitative determination involving no measurement could demonstrate a concept of 'degrees of heat'. Consider Ellis' advice on mashing:

Hot Water or Liquor, for amber or brown Malt, by many is taken three Degrees this Side boiling: That is, when the Water is very hot, and the Fire stirred; if then there arise a white Cloud, it is Time to let it run on the Malt … And for pale Malt the Water is heated to the next higher Degree of Heat, which is when the Water appears in small globular Particles, or little Bubbles which arise on the Top, and then it is just Time to let it run ... ¹⁶

The reference to ‘three Degrees this side boiling’ implies a system of qualitatively differentiated heat states, leading up to boiling point, as follows:

WARNING: list-type attribute on table undefined. Should be one of 'simple', 'decimal', 'bullet' or 'address'

The distinction between the third and fourth degrees is borne out elsewhere in the text by mention of dropping bran in to cleanse the liquor ‘when it is ready to boil or just broke’, and by an otherwise cryptic reference to the second mashing liquor, in porter-brewing, being ready when it ‘is ready to boil, but not boil …’. ¹⁷ We cannot know, however, whether these particular 'degrees' had any generality beyond Ellis' work.

This account also demonstrates that the 'correct' heat state for mashing was understood to be variable - in this case, depending as the malt used was pale (dried with a low heat) or brown (much higher-dried.) Ellis refers briefly at one point to the ‘fiery particles’ of the higherdried brown malt: the underlying assumption, found also in earlier work, is that brown malt contains some persistent principle of fire, and so will require less heat at mashing than pale.^{18 & 19} This perception carries through into Ellis' advice on the proper climates in which to ferment beers brewed from different malts. The spring is suited to pales and ambers, ‘tough Malts, that have the least Share of Fire in them’; October brewing, and a winter fermentation, is recommended for ‘the brown Sorts, whose Bodies are loaded with igneous Particles, and best reduced into a smooth, temperate Condition, by that frigid Season’. ²⁰ This variation, of course, was left entirely to the brewer's personal judgment. Not all pre-thermometric methods were so qualitative. An alternative to the facevisibility criterion was the 'method of mixtures,' whereby cold and boiling water were mixed in set proportion in the mashing vessel. Mentioned briefly by Ellis, ²¹ the method is described in detail in William Reddington's Practical Treatise on Brewing, first published in 1760. Reddington, unlike Ellis, unquestionably had long-term experience as a commercial brewer; his directions were published only posthumously, having been compiled at unknown date (though almost certainly after Ellis's.) He describes the proportions of boiling and cold needed for each mash with considerable precision, reflecting the high degree of volumetric standardisation, and sophistication in gauging techniques, which had entered the brewery even at this early date, chiefly through Excise obligations. The first mash for pale malt, for instance, requires ‘twenty-two barrels and one firkin of boiling Liquor [to] five Barrels, one Firkin, and a half of cold Liquor’: ²² there being four firkins to a barrel, the formula was specified to the nearest eighth of a barrel in a total volume of over 27 barrels. Further mashings had different specified proportions; just as in Ellis' account, the liquors for brown malt are recommended to be cooler than for pale.

Reddington, however, professes no universality for his prescription. He is well aware that local contingencies may vary the result:

Every Brewer should… inspect his Goods, before he takes the next Liquor, that he may know whether he has committed any Error… If part of the Worts only be let out, this can be discovered by squeezing a handful of [the malt]; and if you find [the grains] more greasy and glutinous than common, you should not take the next Liquor so sharp [hot] as usual … ²³

The brewers' senses, then, retained an authoritative role in Reddington's scheme, as in Ellis's and other pre-quantitative accounts. The overall volumes, we should note, are tailored to Reddington's own equipment, and the precision presumably reflects the precision to which his own operations were taken. Other brewers might attain a similar skill, but their figures would not be equivalent to Reddington's: the precision was localised at the level of the individual. The shift towards universal quantitative prescriptions came only with the thermometer - and even here, the brewer's pre-established capacity for judgment was not truly laid aside.

The introduction of the thermometer

The principle of the thermometer was well-established decades before it became commonplace among brewers. Devices of the familiar sealed liquid-inglass form appeared in the seventeenth century; by 1700 thermometry was widely used in experimental philosophy, and increasingly seen as a potential tool for medical diagnosis. It did not, however, signal a 'revolution' in practice: there is significant evolutionary continuity between pre-thermometric and thermometric activity. The 'method of mixtures', for instance, was widely used by experimental philosophers in the early conceptualisation and calibration of thermometric devices, ²⁴ while the thermometric temperature 'degree' can be seen to have evolved from qualitative 'degrees of heat' such as are described in Ellis's account - the familiar degree scale of the instrument- maker Daniel Gabriel Fahrenheit, which became dominant in Britain and the Low Countries around the 1720s, probably developed by subdivision and extension of seven qualitative degrees intended for meteorological work. ²⁵ Some early numerical scales, including one promoted by Isaac Newton, display their qualitative origins by recording varying ‘degrees of boiling’ rather than a single boiling point. ²⁶

The individual usually considered the pioneer of brewery thermometry is Michael Combrune, a first-generation brewer of wealthy Huguenot parentage who developed a moderately successful ale-brewery in Hampstead, then a village outside the metropolis of London, in the 1740s and 50s. ²⁷ Combrune's Essay on Brewing of 1758 is the first known published text to recommend the use of thermometers by brewers. It is also the first brewery publication to make significant use of the language of natural philosophy, and in particular chemistry. In 1762 Combrune followed it with the Theory and Practice of Brewing, an expensive quarto volume clearly modelled on contemporary philosophical treatises, which coupled the theoretical principles laid out in the Essay with detailed directions for thermometric management of the brewing process.

Combrune was probably not the first brewer to consider the thermometer. The instrument had been recommended in hop-drying in 1733, but there is no evidence that it was significantly taken up, or that it passed to the brewery at that time. ²⁸ Combrune suggests in an unpublished manuscript ²⁹ that the instrument first reached the brewers in 1740, and that many practitioners then experimented with it as part of a general push for quality. He adds, however, that most of his contemporaries soon discarded the instrument, not having found its utility. We might speculate that around 1740 some instrument-makers specifically targeted the brewers as a potential market for thermometers, but lacked the specialist knowledge to secure acceptance: this would parallel what later happened with the hydrometer, first marketed to brewers by Benjamin Martin in 1768, but not widely accepted until its operation was elaborated in print by the brewers John Richardson and James Baverstock in the 1780s. ³⁰ In that case, it is entirely possible that a minority of brewers employed the device privately through the 1740s and 50s.

There is no evidence, however, that the device was widely taken up prior to the Essay of 1758; and there is reason enough to believe that Combrune deserves the status of having brought thermometry to prominence. The Essay was dedicated to the Worshipful Company of Brewers, which at that time had lost its power as a regulatory authority but maintained its role a social sphere; knowledge of the publication would have spread quickly and generally among the London commercial brewery. Further, at a time when much of the reviewing press attempted to provide notices of all new publications, the Essay received extensive reviews in journals including the Gentleman's Magazine, Monthly Review and Critical Review. These, although decidedly mixed in their reception of Combrune's efforts, would have spread the word to a gentlemanly audience nationwide, including not only common brewers in other towns, but also the rural gentry who continued to brew large quantities for private use.

It is important to note, as I discuss at length in another paper, ³¹ that Combrune's aim was not simply to promote thermometric measurement in place of the old, qualitative criteria. Combrune's writings were heavily influenced by Herman Boerhaave, then the dominant Continental authority in chemistry and medicine, who had been one of Fahrenheit's most influential clients and enthusiastically promoted the diagnostic use of his products. Around 1760 Combrune was in correspondence with the lecturing chemist and physician Peter Shaw, whose unauthorised translations had been most influential in bringing a version of Boerhaavian theory to Englishspeakers; Shaw may have been Combrune's friend or intellectual patron, helping his works to secure an audience. It is sometimes assumed that Shaw introduced Combrune to the thermometer, ³² but this is unlikely: by his own account, Combrune began to experiment with the device in 1741, ³³ and derived some of his knowledge of Boerhaave and of the Fahrenheit thermometer from other sources. ³⁴

To Combrune, the thermometer was the central diagnostic tool in a grand theory of brewery management based on Boerhaavian acid-oil chemistry. In fact the Essay barely discusses the quantification of mashing heat, being more concerned with the nature of malts: those dried at low heat (around 120 degrees Fahrenheit), believes Combrune, are full of acid principles, tending to produce an accelerated fermentation which can easily run out of control, leading to putrescence; but as the malting temperature is raised, oily principles are drawn out which cause the acids to be ‘smoothed over’, producing sweet fermentables. ³⁵ Thus, Combrune reasons, the malting heat determines the rate of fermentation and hence the time taken for the beer to come into, and to fall out of, drinkable condition. The 1762 Theory and Practice modifies this scheme to give a role to the mashing temperature, and also to the hops used. The result is a simple arithmetical relationship. Using an empirical table provided by Combrune, the required maturation time for any intended beer is given an equivalent Fahrenheit value. Another empirical value is found for the effect of the hops, and the drying heat of the malt to be used is estimated from its colour. The average of the mashing heats to be applied for the desired product can then be found as follows:

mean mashing heat = [2 × ( maturation value - value of hops )] - malt-drying heat

Further theoretical considerations are applied to establish the spread from highest to lowest mashing temperature. Although very abstruse, and employing many ad hoc corrections, Combrune's reasoning results in concrete figures: pale keeping beer, for instance, requires a first mash at 138° and a final mash at 164°. For porter, brewed with higherdried malt but requiring a longer maturation time, the first mash is at 144° and the last at 162°. ³⁶

The relationship illustrates an important point. As noted earlier, brewery tradition had it that heat (‘fiery particles’ in Ellis' terms) was impressed upon, or stored within, the malt in proportion to the heat applied to dry it; consequently, the higherdried malts would require lesser mashing heats. Combrune dissents from this position, but his method conspicuously preserves its operational consequences: for any given maturation value, higherdried malt is best managed by a cooler mash. At the same time, recapturing the traditional relationship did not mean that Combrune was obliged to follow it. By his formula, it holds only so long as the maturation value (and thus the intended mode of brewing) is unchanged: thus, both the malting and mashing heats are higher. Neatly, several of the identifying features of London porter (heavy hopping, long storage, artificial fining) were bound up in the maturation properties which Combrune appointed to make this possible. Some aspects of porter's identity were merely a few years old when Combrune began his researches; ³⁷ perhaps, then, his scheme was deliberately developed to accommodate newlydeveloped urban practices which seemed to go against traditional wisdom.

The spread of thermometric practice

As Mathias has pointed out, ³⁸ there is very limited evidence from which to gauge the spread of thermometers in the brewery even after Combrune's publications: the devices were small, inexpensive compared to the outlays normally recorded in brewery ledgers, and may in any case typically have been the personal property of operative brewers. The author of Every Man His Own Brewer, a heavily Combrune-influenced manual published anonymously in 1768, suggests that the device had been universally adopted in the commercial London porter brewery by that date. ³⁹

Broadly, it seems that thermometric practice spread from London to the provinces and from the larger to the smaller common breweries, reaching the publican-brewers later and private families last of all; there must, however, have been some complexity to the process. Common breweries, though they had the incentives of cut-throat competition and rapid information networks, also had a strong tendency to conservatism in processes which had, in many cases, established large fortunes (compare Whitbread's initial rejection of the hydrometer). ⁴⁰ Correspondingly, some smaller or private brewers must have adopted the novel device soon after hearing about it. John Richardson of Hull, London-trained and author of some of the most zealously thermometric brewery texts, suggested the device was becoming general among common brewers in the 1780s, against the will of brewery servants who saw it as a potential ‘rule in the master's hand’. ⁴¹

William Ploughman suggested the thermometer was only ‘now getting into general use among public brewers’ towards the end of the century, but felt it would be ‘too expensive’ for private families, recommending the method of mixtures instead. ⁴² From around 1790, most texts at least mention the thermometer: the handful which do not are generally aimed at the domestic brewer. Pamela Sambrook's extensive survey of countryhouse brewing ⁴³ suggests that the spread to private houses was very slow, with the device rarely seen before the mid-nineteenth century; Sir Thomas Broughton's butler, Shore, was ‘unusual’ in using one in 1804. Shore stated that ‘great variation in the produce’ of his own operations had led him to introduce a thermometer of the kind essential in ‘large brewings’, but otherwise endorsed the method of mixtures. ⁴⁴ In 1829 the chemist David Booth said the thermometer was ‘unknown to nine-tenths of the private brewers’; ⁴⁵ yet mass-produced devices were becoming increasingly attainable. A few years later William Chadwick, a butler writing for a domestic audience, saw no reason why a private brewer should not possess both a thermometer and a saccharometer. ⁴⁶

Combrune's quantitative chemical management scheme, and its grounding in Boerhaavian theory, found support among several later writers.^{47, 48, 49 & 50} These, however, were distinctly in the minority: more usually, the thermometer was adopted, but the theory Combrune had nominated to underpin it was ignored. Combrunian chemistry represented only one possible interpretation of the thermometer - which was, of course, a long-established philosophical instrument, over which Combrune could exert no proprietory control. The thermometer and its 'standard' Fahrenheit degree scale could in fact be applied in an intensely local manner. It is notable that the actual operation of the thermometer was heavily underdetermined by Combrune's and most other accounts: procedural descriptions covering the positioning of the bulb, the time the instrument should be left to stand before taking a reading and so forth are exceptionally rare (two such occur in the anonymous Theory and Practice of Malting and Brewing of 1805, and the later work of John Levesque).^{51 & 52} Brewers skilled in the use of delicate equipment, and inured by the unloved Excise to endless gauging and tabulation, would surely have found their own means to apply the thermometer without such precise directions.

This may explain why established sequences of operations were not, apparently, affected at first by the coming of the thermometer. The author of Every Man His Own Brewer - who professes himself a ‘Gentleman lately retired from the brewing business’, but discusses the operative brewers in the porter brewery of London as a group distinct from himself - grumbles at the unchanged convention of setting mash temperature for table beer by first boiling the liquor, then cooling it to the desired temperature (now verified thermometrically) by the addition of cold. ⁵³ To the author, the thermometer represents a means to do away with the ‘double trouble’ of heating and cooling, by simply heating no higher than the desired value in the first place; the generality of commercial brewers, however, cleave to a pattern which recaptures the operations of the prethermometric method of mixtures.

Another impression of the infiltration of thermometry into brewers' practice may be gained from a well-preserved account surviving in manuscripts from the Bankside Distillery, London, covering the period May 1765 to June 1772. ⁵⁴ In a gradual transition, the unknown author or authors begin to incorporate temperature readings for the heat of the mash used to prepare wash, which had to be steered between inefficiency and 'setting' just as in the brewery. At first, only the heat in the underback, after mashing and draining (potentially significant to the progress of the fermentation) is recorded thermometrically: the mashing water is characterised by the quantities of ‘Hot Gallons’ and ‘Cold Gallons’, indicating use of the method of mixtures. These volumetric figures are retained throughout, but from October 1768 there is an additional column which is ultimately identified as ‘1st Mash Heat’. Whereas the underback heat varies between 36 and 51° (clearly Fahrenheit), the mashing heat varies only from 136 to 138°: clearly, although the mash liquor was still mixed from hot and cold water, it was now being positively set by reference to the thermometer, ironing out the effects of seasonal variation.

Brewers' thermometers usually differed in their material construction from the devices wielded by chemical philosophers. Although Combrune and others doubtless made their early experiments with general-purpose philosophical or medical instruments, the aforementioned London maker, Benjamin Martin, refers with little elaboration to the ‘Brewer's Thermometer’ as early as 1766: ⁵⁵ this suggests that a specialised instrument, perhaps larger and harder-wearing than normal, was established within a decade of Combrune's Essay. In fact, there were several different adaptations, owing to the range of operations and working scales involved in the brewing process. David Booth describes ‘tun-thermometers’ particularly adapted to the primary fermentation, when a huge cloudy head covered the wort: the instrument was at least three feet long, so that, with the bulb immersed in the liquid, ‘all that is necessary of the scale overtops the froth of the head’. Booth outlines the possibility of a specific thermometer for the copper, with a distinctive marker which would show the temperature clearly through the fog of condensing steam, and it seems that at least one maker, William Loftus, developed such a device.^{56 & 57} In the 1830s, William Chadwick suggests even the private brewer should acquire ‘a second thermometer, with a tin guard, to use in the boiler’. ⁵⁸

Of particular interest is the 'blind' or 'private' thermometer. This device, commercially available by 1802, featured a detachable scale: the body of the thermometer bore only a travelling index marker. This marker would be set to a particular value, such as a mashing heat, with the scale in place; with the scale removed, the thermometer could be applied to set this heat without the privileged value being discernible by anyone - including the brewery servant actually performing the operation, an important consideration in competitive markets with a high turnover of staff, where not only ‘impertinent pryings’ but serious industrial espionage was a real possibility.^{59, 60 & 61} The use of this safeguard seems to have been a common practice for decades. In the 1850s and 60s, Loftus ⁶² advertised a fourteen-inch ‘Blind Scale’ thermometer at sixteen shillings, two shillings dearer than his similar fixed-scale device, while Oswald Pearce Serocold, of the large London firm of Reid's, described from personal experience the brewery culture of the 1880s as follows:

Brewing operations were veiled in secrecy, so much so that the thermometers used for determining the heat were not marked in degrees but by letters, and a loose scale marking true degrees had to be applied to obtain a correct result. Even entries in brewing books were partly faked so as deliberately to deceive the casual reader! ⁶³

Something similar probably occurred at Reid's rival Barclay Perkins: the brewing book for 1812-1814 opens with a diagram of a thermometric scale, against which letters are marked alphabetically at ten-degree intervals, A standing for 30°F, B for 40° and so on up to T for 210°. ⁶⁴ Even this simple scheme could have been sufficient to confuse outsiders, although the book itself seems to contain straightforward Fahrenheit values. Blind thermometers and value encoding serve as useful evidence of a form of technological innovation independent of naturalphilosophical or scientific considerations.

Changing perceptions of thermometry

We have already seen that Michael Combrune's chemical scheme of thermometric management did not survive among the brewers who adopted the device; we should, therefore, not be surprised to find the meanings associated with it changing extensively through the late eighteenth and early nineteenth centuries. Particularly influential was the popularisation from the mid-1780s of brewery hydrometers and saccharometers, allowing the gravimetric determination of original gravity, and hence of the extractive potential of malt samples. The two principal apostles of this process, James Baverstock of Alton and John Richardson of Hull, were both early and enthusiastic adopters of the thermometer; but, with the device well-established by the time of their principal publications, ^{65 & 66} they inclined more to stress its role as an adjunct to hydrometric measurement. Specific gravity values were temperature-dependent, and needed to be quoted alongside a temperature value; often, the sample would be cooled to a reference standard temperature before recording.

Richardson, in particular, was concerned with conceptual territory. Unlike Combrune, he marketed a proprietary device through a monopoly agreement with a London maker, presenting it as a wholly new and brewery-specific invention rather than a familiar philosophical instrument. ⁶⁷ A key object of his scheme was to establish the saccharometer as the sole indicator of wort quality; this required the displacement of the thermometer from that position, where Combrune had aimed to place it. ⁶⁸ Although Richardson must have been aware of Combrune's research, Combrune's name is nowhere mentioned in his account, and the principles underlying his theory are quietly unpicked.

Richardson did agree with Combrune that the most significant advantage of the thermometer lay in allowing the adaptation of the brewing process, and in particular the mash, to different purposes and varieties of malt. ⁶⁹ This did not need to be an especially precise process: neither Combrune nor Richardson would have seen value in recording values more closely than to a degree Fahrenheit. Yet the introduction of hydrometers had the additional effect of focusing brewers' attention on the developing Excise project of gravimetric determination. Although the beer trade was not actually taxed on the basis of gravity until 1880, the principle was well-established in the distillery a hundred years earlier; the Commissioners of Excise worked closely with instrument-makers and analysts to be able to present their judgments as precise and reliable, so ensuring their authority. ⁷⁰ The distillers, and ultimately the brewers, may not have required high levels of precision for their own practice, but understood the need to emulate the techniques and rhetoric of the Excise to safeguard their own credibility.

Brewers of the nineteenth century increasingly investigated emerging scientific practice, and alongside the gravimetric quantity the 'standard heat' became a brewery convention. ⁷¹ As the Excise developed analytical laboratory facilities and promoted scientific training, so increasingly did the breweries.^{72 & 73} This trend, arguably, led to the achievements of the Salford brewer and thermodynamicist James Prescott Joule; the concern with absolute standards and attunement to instruments of precision which Joule obtained through exposure to the new agenda of the brewing profession, argues Otto Sibum, made possible the exceptionally precise operations which underpinned his quantification of the mechanical equivalent of heat. ⁷⁴

Another change was set in motion when, in establishing their individual modes of management, brewers discovered the intense spatial and temporal localisation of thermometric values. This factor which challenged existing sensory understandings: whereas the brewer's traditional experience told only that a mashing, or gyle, or fermentation vessel, was (or was not) in a 'right' heat condition, the thermometer could give a vastly more complex interpretation, showing heat gradients across large vessels and minute, unpredictable variations in fermenting temperature. One response, as we have seen, was to preserve pre-thermometric practices, which had the advantage of being tried and trusted: doubt as to the constancy of the thermometric quantity appears to have been the principal justification of those brewers who continued to cool their mashing liquor from boiling. ⁷⁵ An alternative response, especially at larger scales of operation, was to attemperate: to introduce heating and cooling equipment to impose thermometric constancy.

In 1790, John Long of Dublin patented a temperature-control system based on submerged coiled pipes, applicable to mash-liquor and fermenting worts.^{76 & 77} Richard Shannon, who had become acutely concerned with thermometric issues through a reading of John Richardson's works, suggested in 1805 that the cooling ‘worm’ was already in widespread use: he marketed his own ‘refrigeratory’ on this principle, along with a ‘cooling attemperator’, a fanning device claimed to allow slower and more controllable cooling, as might be required during the sensitive cleansing phase. Stabilisation achieved using the thermometer, rather than straightforward cooling, was stressed in Shannon's account. ⁷⁸ The later account of John Levesque, who marketed his own instruments, stressed the ability to ‘affix’ certain temperatures, indicating that the nineteenth-century thermometer was used less for diagnosis than for the verification of intended results. ⁷⁹ This position is well summarised by W.L. Tizard, who marketed a mashing attemperator with heating pipes in the hollow shaft and rakes:

[T]he use of the thermometer, as a standard of temperature, must be strikingly obvious; but when we consider it as an implement of science and not of art, as a theoretical test and not as a problematical means, as an indicator of heat and not the cause of it, we have attributed to it all that belongs to it: but the brewer wants more; he wants, in fact, to attemper his heat to his subject, and he wants a thermometer to prove that it is properly attempered. ⁸⁰

The very largest concerns, such as Whitbread's London porter brewery, made extraordinarily systematic use of this mode of thermometry in order to regulate bulks of liquid which would otherwise have been highly unpredictable. Thermometers were dispersed through the plant's network of piping, and, depending on the readings, cocks could be switched to send the cold supply from the brewery's main well through a cooling jacket over the fermentingsquares' supply pipe. ⁸¹ Attemperating innovations gradually reduced and eventually abolished the summer period traditionally held to be too unpredictable for brewing operations to be carried on, allowing all the cost advantages of a continuous supply. The brewing and malting instructor Thomas Hitchcock asserted refrigerators to be ‘indispensable in a public brewery’ in 1842; ⁸² certainly, rigorous temperature control was general throughout the larger commercial breweries by this point.

The thermometer, then, was no longer a diagnostic implement, but a rule against which materials and equipment could be standardised. It is probably not overstating the case to say that temperature control was central to the shift from traditional porter production to large-scale use of the more delicate ale-style methods in London, and, consequently, to the rise of mass pale ale production in Burton and other provincial sites and the eclipse of London as a major beer-producing centre - surely the defining event in nineteenth- century British brewery culture.