Can one use a Styrofoam cup as a Keli Sheni or Shlishi? Styrofoam doesn’t cool down the liquid very much.


It is considered a Keli Sheni (or Shlishi, when using two cups).

However, it should be noted that one should be stringent with boiling water in a Keli Sheni, and it should be considered as Keli Rishon. The solution would be to pour from the Keli Sheni cup into another cup and only pour from the second cup into the bowl or cup. In this way, the cold contents of the bowl or cup serve to cool off the hot liquid.

Additionally, there are those who question the acceptability of Keli Shlishi altogether, see here (in the references):

Is it permissible to make tea and coffee in a Keli Shlishi using instant coffee or tea bags, or is this considered coloring the water? 



בענין קלקר או תרמוס – רוה״פ נקטו שדינו ככ״ש. וראה שלחן שלמה שיח, מב בסופו. וראה שם מז, ב. הנסמן בשו״ת נשמת שבת ה יא. שבת כהלכה א, א הערה צג. אליבא דהלכתא גל׳ סא ע׳ לח. שם ע׳ פו.

בחומרת אמבטי – כ״כ בחי׳ הצ״צ שבת פ״ג – לט, סע״ב. וראה הנסמן בשבת כהלכה שם לח ואילך.