Lattice bwplot(): Conditioning on one factor

I'm not able to create the proper syntax to specify a lattice bwplot() for
only one of two conditioning factors.

   The syntax that produces a box plot of each of the two conditioning
factors is:

bwplot(quant ~ param | era, data=mg.d, main='Dissolved Magnesium', ylab='Concentration (mg/L)')

   What I've tried unsuccessfully are:

bwplot(quant ~ param | factor(era=='Pre-mining'), data=mg.d,
main='Magnesium', ylab='Concentration (mg/L))

bwplot(quant ~ param | era, data=mg.d, main='Magnesium', ylab='Concentration
(mg/L)', subset=era('Pre-mining'))

plus slight variations of the above. None work.

   Please point me to what I've missed in specifying only one of two
conditioning factors for the plot.

Rich