It was a big week in New York last week. More than $1.2bn in Contemporary art was sold representing a rise of 6.7% over the previous year. The Evening sale portion of that total rose at a greater rate than the day sale portion. The Evening sales were up by 7.8% while the day sales total increased by only 2.5%.

That disparity was a product of the very top end of the market. In the Evening sale this May, 46% of the $980m total was comprised by the top ten lots for $461m. Last year, that comparable figure was only 36%. Very strong performances for lots by Jeff Koons, Robert Rauschenberg, Francis Bacon and Mark Rothko were the heart of that story. When very expensive lots get bid upon aggressively, we should conclude that we've seen a shift toward the top of the market. Although the numbers suggest this is more driven by supply—meaning the demand emerged based upon the quality of the lots—than it is by blanket demand or an overall wealth effect. (There's more to say about this when we get to the top Evening sale lots below.)

Curiously, sell through rates were slightly lower this year over last year which may reflect consignors' greater willingness to bear risk or even keep their art even in the face of bidding close to their reserves.

Sotheby's had a very strong $341.8m Contemporary art Evening sale filled with surprises. The two top lots were sold for $50m prices against estimates that were a fraction of that price. In the case of the Francis Bacon "Screaming Pope" from the Lang estate, the bidding war was expected barring some sudden collapse of the market. Widely viewed as one of the best examples of this coveted part of Bacon's oeuvre, the painting's small size dictated a conservative estimate that was likely to see strong bidding. Still, $50m was the kind of price that requires more than hope to achieve.

San Francisco MoMA's 1960 Mark Rothko painting was originally viewed as aggressively estimated when Sotheby's first announced it would come to the block with a $35m price tag. An irrevocable bid was an additional indication that the work might have been priced to perfection. There was even an algorithm that claimed to be able to predict the final price at 15% below the final price Sotheby's was able to elicit from the buyer.

The success of those two works stands in contrast to seven of the other eight works in Sotheby's top ten which were sold for prices within the estimate ranges. There were plenty of other occasions for fireworks in this sale but they took place lower down the price spectrum further differentiating Sotheby's from its main rival.

We'll have more on the sales composition in our analysis next week. In the meantime, let's look at how this sale stacks up against the last 12 years of May sales.

Last night's PWC Evening sale at Christie's might not have been the biggest May Evening sale but it was something of a dramatic turn from previous years both in the absence of works offered with estimate upon request and for the volume of art that traded hands with a total just behind the peak years of 2014 and 2015.

The top-heavy sale, three works were offered with low estimates of $50m, had 70% of the value in the top ten lots, was notable for the number of works sold, 91%, and the greater value of the sale offered with a financial backing of any kind.

Sotheby's Impressionist and Modern Evening sale came in slightly above the previous year's sale but for a number of internal reasons, it was far stronger than that sale. The two events represent a sea-change in Sotheby's Imp-Mod performance in May with totals approaching peak years like 2012 and within striking distance of the top in 2015.

The sales total was very top heavy with the top ten lots representing 70% of the total and more than 50% of the total coming from the top three lots. That highlights what a selective market Imp-Mods is but these results are better because they don't rest upon third-party guarantees that evinced no competition.