For softer cores John, add less Sodium Silicate. 4% by weight at the top end working down as low as 1% for really friable ones. For easier breakdown post casting add icing sugar to the core mix.
Years ago when I started using Sodium Silicate I made a series of test cores. I made a 8% mix, made a core, mixed in more sand to reduce the % sodium Silicate, made another core etc each time using digital kitchen scales to accurately assess the %.
The ideal does vary with the sand grain size and make up of the sodium silicate (which oddly isn't a specific composition but a ratio)