var pictureClasses=new Array(".material-plastic-alt", ".material-aluminum-alt");
var picturesNumber=pictureClasses.length;
var pictureCounter = 0;

function fadePicture(identifier) {
	var identifierObj=$(identifier);
	if (identifierObj.css("display")=="none") {
		identifierObj.fadeIn(1000);
	} else {
		identifierObj.fadeOut(1000);
	}
}

function alternatePictures() {
	fadePicture(pictureClasses[pictureCounter]);
	pictureCounter=(pictureCounter+1)%picturesNumber;
}

$(document).ready(function() {
	setInterval(alternatePictures, 3000);
});